Paul Simonon meets me outside Westbourne Park tube station on a cold, bright March morning, and we walk to his studio. It stands right underneath the Westway, the sweeping brutalist concrete flyover that the Clash hymned on 'London's Burning', one of several short, urgent songs on their eponymously titled debut album from 1977.

Another song from that time, 'White Riot', was inspired by the violence that broke out on nearby Ladbroke Grove after the Notting Hill Carnival in 1976. I ask Simonon if it feels strange to walk through his own history, his own mythology, every time he goes to work.

'Most of the time, I don't really think about it,' he says, grinning his endearing, gap-toothed grin as we stand outside his padlocked studio, and stare up at the monolithic concrete structure that looms above us. 'But I guess it's kind of poetic that this is where the Clash started out and this is where I've ended up.'

When the London punk scene began in 1976, Simonon was a fledgling painter, fresh from Byam Shaw art college which, back then, was just up the road in Notting Hill. In the spirit of the times, he bought a bass guitar which he drip-painted in the style of Jackson Pollock and learned how to play by writing out the chords and sticking them on to the instrument's neck.

Thirty years on, he describes himself as 'a painter who occasionally dabbles in music'. His most recent bout of dabbling, though, led to a number one album as part of the Damon Albarn-orchestrated supergroup, the Good, the Bad and the Queen. 'It's done and dusted,' he says of that project, but later lets slip that the group are in negotiations to play a big benefit for the newly reignited Rock Against Racism campaign. The gig is scheduled for 27 April in Victoria Park, east London, where, 30 years ago, the Clash rocked against racism before 100,000 people.

'I can dip in and out of music when I feel like it,' says Simonon, 'but it's not my life any more. There was a point after the whole intensity of the Clash finally subsided when I just found that painting grounded me in a way that music didn't.'

Simonon has been painting seriously, quietly and determinedly, since 1986, when he returned from a stint in LA and was startled by the British weather. 'I saw the beauty in it for the first time - the clouds, the rain, the cold and even the grey skies. It suddenly seemed rich compared to the sameness of Los Angeles. I went out in the rain and drew the gasworks by the canal. That was the turning point.' He also started visiting museums again with his sketchbook, drawing 'faces, hands, feet, whatever took my fancy'.

In 2002, he had a show, From Hammersmith to Greenwich, at a posh gallery in Green Park, featuring several big London riverscapes, each of which sold for around £4,000.

Later this month, at Thomas Williams Fine Art on Old Bond Street, he will unveil his most recent work, a series of figurative paintings inspired by the bullfights he witnessed in Madrid in the summer of 2003, as well as some small still lives.

'Paul is not in any way a dilettante,' says his gallerist, Thomas Williams. 'He has the dedication of the true artist. He lives and breathes art, and is constantly thinking and talking about painting. We already have an incredible amount of interest in the show.'

That interest is reflected in the prices which, this time around, will range from £5,000 for a small work to £30,000 for one of the larger canvases. 'As far as I'm concerned, his musical career was a brief interlude in his artistic one,' says Williams. 'He's an artist. It's the music that's the aberration, not the painting.'

Nevertheless, at 52, Simonon is a rock legend, revered by younger musicians who still see punk - and the Clash in particular - as a template for rock'n'roll rebellion. Always the most dapper member of the group, he remains a snappy dresser, decked out today in a tailored coat, pinstripe suit and silk scarf. When he smiles, you can see traces of his younger self, the angular cheekbones and sullen good looks that made him punk's premier pin-up. He has long since shed the rude-boy attitude that was a prerequisite of those angry times, but still looks like he could hold his own should the circumstances arise.

These days, though, people tend to want to shake his hand rather than throw a punch at him, like they did back when punk gatecrashed the mainstream following the Sex Pistols' expletive-strewn appearance on prime-time TV in the summer of 1977. 'It was intense back then,' he nods, when I mention how violent a place Britain was in the late Seventies. 'People wanting to fight us, jumping on stage to punch us. If you had short hair and looked at all like a punk, you wouldn't get served in many pubs. Then, you had the Teds, who really took it all personally. I remember walking down Shaftesbury Avenue with a girl, and seeing this blur of movement out of the corner of my eye. It was this big Teddy Boy running through the traffic to have a go. Mad.'

As it happens, I had bumped into John Lydon, formerly Johnny Rotten, in Soho just a few nights previously. I tell Simonon that Lydon was standing outside the Groucho Club with Chas Smash from Madness, surrounded by homeless guys singing 'God Save the Queen'. Simonon laughs. 'So, John's in town,' he says, in a way that suggests Lydon's reputation for inciting chaos remains intact. 'How is he?' Same as always, I say - still raging, still pontificating. 'I always got on with John,' muses Simonon, smiling, 'We came from the same place - non-musicians. We saw our chance and we took it. The thing is, I always had the painting to fall back on. That's what pulled me through the so-called wilderness years. I never wanted to go back and relive the glory days, I just want to keep moving forward. That's what I took from punk. Keep going. Don't look back.'

According to Chris Salewicz, author of Redemption Song, the recent biography of the Clash's lead singer, Joe Strummer, Simonon is 'essentially quite a shy bloke, but also a bit of a prankster'. He reminds me that, back in the early days of the Clash, it was Simonon who made it into the tabloids when he was arrested for shooting pigeons with an air gun from the window of the group's Camden Town rehearsal studio.

'Paul was a bit of a bad boy back then,' says Salewicz, 'but that may just have been what was required at the time. The way he has managed to move from rock'n'roll back to painting with such ease is quite remarkable in itself when you consider how diametrically opposed the two are in terms of artistic endeavour. Then again, he has always been self-contained; someone who, you suspect, is very much at ease with himself, and with the solitary nature of painting. He's definitely a bit of a thinker.'

Simonon's studio is brightly lit, cluttered but comfortable, a pair of two-bar electric fires throwing out some much needed heat. There are newly finished canvases stacked in rows, tables full of reference material - postcards, books on bullfighting and art, holy pictures. One wall is partially obscured by stacks of boxes from his recent house move. A bottle of Whyte & Mackay whisky and a jar of instant coffee stand side by side by a kettle for those winter mornings when he needs a kickstart.

Underneath the light streaming in from the high windows stands an easel on which a nearly completed painting rests. It depicts a huge bullring and a parade of toreadors. Like his Thames paintings, the scene has been painted from life, and from somewhere high above the action. Simonon is essentially a figurative painter, concerned as much with the application of paint as with the subject. It is obvious from the work, and the real sense that this is a working studio, visited daily, that he is not just another rock star who paints as a hobby.

'I want nothing to do with all that stuff,' he says, settling down with his Scotch and coffee. 'I'm not mentioning any names, but most so-called art made by rock stars is fucking dreadful.' Simonon's paintings, just in their painterliness alone, seem like they belong to another time. 'Paul's not a conceptualist who parades his intellectual pretensions,' says Williams. 'He really belongs to an older English tradition, to Augustus John and the Edwardians.'

Even as a teenage art student, Simonon had little interest in being contemporary or cutting edge in his painting, preferring the likes of Constable and Sickert to Warhol and De Kooning. He won a scholarship to Byam Shaw but lasted a year-and-a-half, dismayed by the teachers' total espousal of American abstraction. He points to a painting on the wall of his studio, an angular urban landscape that, were there elongated figures in it, might have been painted by Edward Burra.

'That was the last painting I did at art school. The students loved it but the teachers hated it. I'd had enough by then.'

Simonon's traditionalist approach to painting is surprising given that, within the often volatile creative dynamic of the Clash, he was the conceptualist, the one who paid most attention to the visuals, the image. He painted the backdrop to the Clash's rehearsal studio, and designed some of the later stage sets, including the dive-bombing Stukas that echoed their often explosive performances. You could tell the Clash were art-school punks from the start, what with those shirts stencilled with slogans and that paint-splashed bass guitar.

'That was the art student in me trying to find a look that would make us stand apart from the Pistols,' he says, laughing. 'The Buzzcocks were very Mondrian, and we were Pollock. As a painter, though, I'm essentially old-fashioned. Conceptualism just doesn't do it for me. I love Walter Sickert, Samuel Palmer, Rubens and Constable. That's just the way I am. I love putting paint on canvas, getting lost in the process of painting.'

Like many painters, Simonon has a passion for the sheer physicality of the job. For his Thames series, he carried his huge canvases up on to the roofs of various high buildings along the river and often had to rope them to railings to stop the wind carrying them off.

'I was on top of the Shell Mex building for weeks,' he laughs.

'I think I entered a trancelike state up there. Bit like Blake. Hours would go by and I'd suddenly realise I was bloody freezing.'

He'd read somewhere that Jeffrey Archer, then still a Tory MP, had an apartment overlooking the Thames, so he wrote to him, asking if he could paint the river from his balcony. 'He wrote back and said OK,' says Simonon, grinning. 'I was there for a week. I think he got a bit pissed off with this hulking great bloody canvas in his kitchen every morning, but, I have to hand it to him, he didn't go back on his word and chuck me out.'

I ask if he can remember the first painting he did which he felt happy with. 'Not exactly,' he says. 'I was always painting what was around me, though. As a kid, I always wanted toy soldiers to play with, so I used to paint them on pieces of paper. Create them for myself. In a way, it's still the same. Whether I'm in a good mood or a bad mood, painting takes me out of myself. And I've realised lately that it often resolves things for me.'

Of late, there has been quite a lot to resolve. Towards the end of 2006, Simonon found himself in the tabloids after news broke that he had split up with his wife Tricia Ronane, and began a new relationship with their friend, Serena Rees, co-owner of Agent Provocateur. To complicate matters further, Rees was married to Joe Corre, her business partner and the son of Vivienne Westwood and Malcolm McClaren, the architects of punk outrage, whom Simonon has known since 1976.

An intensely private character, Simonon steadfastly refuses to talk about his personal life, but he seems remarkably content despite all the upheaval. 'It's not something I can or want to talk about, except to say that I'm happy,' he says. 'My kids are growing up. Claude is 14, Louis is 16. We have a good relationship, I have my paint and my life is generally good. Full stop.'

Paul Gustave Simonon was born in Brixton, south London in 1955, and grew up, as he puts it, 'all over the place - Brixton, Ramsgate, Canterbury, Thornton Heath, Bury St Edmunds, Ladbroke Grove'. His mother was a librarian and he describes his father, Gustave, as 'a Sunday painter. Literally.' Simonon senior also seems to have been quite a character. He went AWOL from the army having served in Kenya during the time of the Mau Mau rebellion. 'I think he saw some bad things,' says Simonon, 'and was haunted by them for a long time afterwards.'

Simonon's parents split up when he was 10, and he lived for a year in Rome and Siena with his mother and stepfather. 'It was all a bit bohemian, wandering around these beautiful streets by day, and being taught by my mum for a few hours in the evening.' In his teens, having returned to Brixton, he ran wild for a while with a gang of skinheads before ending up sleeping on the floor of his dad's studio in Ladbroke Grove. 'It was a bit Steptoe and Son,' he says, 'but with discipline. He set me homework, made me paint every day. I learnt the technical stuff from a mate of my dad's who knew how to do glazes and underpainting. It was invaluable, really. He'd hand me a brush and go, "Here, Paul, I have to go to work, finish off that fox for me.'"

Simonon was raised a Roman Catholic but vividly remembers his dad arriving home from work one day and announcing his conversion to Communism. 'He suddenly became a Marxist,' he says, looking like he still hasn't quite got over the shock. 'One minute I was making a papier-mache crucifix scene for Easter, the next I'm selling pamphlets to help liberate the workers. It was all a bit extreme.'

For all the domestic upheaval and constant flitting from place to place, Simonon seems to have come through remarkably unscathed. He is certainly the most grounded punk survivor I have met in a long time. 'I suppose my upbringing made me resilient in some way,' he says. 'What I remember most, though, is that feeling of always being the new boy at school. That was kind of tough. I have absolutely no friends from school, no connections from back then. I was always moving on. I gained a certain independence from that experience. Funnily enough, all the members of the Clash had it, too. We all came from broken homes - even our manager, Bernie Rhodes.'

Back in 1976, it was the wily Rhodes who instructed Mick Jones to recruit Simonon to the group that would soon become the Clash, simply because he looked the part. 'I was a bit Bowie, a bit suedehead back then,' says Simonon. 'And, more importantly, I was at art college. Mick liked that. He was always big on pop history. He knew all about Stuart Sutcliffe, who was Lennon's best mate in the early days of the Beatles, and a proper artist. I remember Mick introducing me to all his mates: "This is my new bass guitarist, Paul. He can't play but he's a painter."'

The rest, as they say, is rock'n'roll history. Together, at Rhodes's urging, they recruited Joe Strummer to the cause, and the Clash became the coolest punk group on the planet and, after the Sex Pistols imploded, the biggest British rock group since Led Zeppelin. Simonon only wrote a few Clash songs, including the somewhat misguided 'Guns of Brixton', but his reggae-influenced bass playing was integral to the group's sound. He was 21 when he joined the Clash, and 31 when they split asunder for the final time in 1986. It was, he says, 'a heightened state of alert throughout. A total life experience. It was working and living and being the Clash, 24-7. We had to live and be it all the time, and we struggled to find what it was every single day.'

Does he have any regrets that the group split just as America was ready to fall at their feet? 'Not really. I mean, it would be nice to have loads of money, but that was never a consideration back then. It was always very fast-forward, let's keep the urgency.'

That urgency was dissipated somewhat by the constant rows between the band members as their popularity grew and the original punk principles became impossible to live by. Mick Jones was the first to go, sacked in 1983 by Simonon and Strummer after one row too many. A new version of the Clash continued for a while but it simply wasn't the same. Then, as suddenly as it had started, the 'heightened state of alert' ended, and Simonon found himself sitting at home, bored and brooding.

'It was a weird time,' he shakes his head. 'I guess I was a bit dazed. Bewildered. The thing was, Mick lived just around the corner and he had formed Big Audio Dynamite. I'd see his tour bus heading off, and I wasn't going anywhere. It was tough, after all those years of the constant Clash agenda. That's really what sent me back to painting. It was the only thing that kept me sane.'

Treading carefully, I ask him how he coped with the news of Joe Strummer's sudden death back in December 2002. He stares at the ground. 'Well, me and Joe were tight, you know. We were very close throughout the early days, living on the street, sharing dole money. And then he stayed at my house a lot after the band broke up. So it was tough. Really tough. First, it's like you are shocked so much you don't even know you're in shock. Then, you have to find some way of making sense of it.'

Has he made sense of it yet? 'I think so. Yeah. Finding out that Joe had a congenital heart condition helped, in an odd way. I mean, it could have happened at any time along the way. It's great in a way that he crammed so much in. He used his allotted time to the full.'

The day before Strummer died, he sent Paul Simonon a text message. It said, 'Come on, Paul. Give it a try. You might even like it.' Strummer was referring to a possible reunion of the Clash for a one-off gig to celebrate their imminent induction into the Rock and Roll Hall of Fame. Simonon, though, was having none of it. 'Joe was up for it, and so was Mick and Topper [Headon, the Clash's drummer], but I wasn't,' he says, without a trace of regret. 'I was the one who always said no. In this instance, I really didn't believe it was the right moment. A big corporate event like that, two grand a seat. Nah, that wasn't in the spirit of the Clash, was it?'

In the end, he went to the Hall of Fame ceremony to celebrate Joe Strummer's legacy and to support his widow, Lucinda. 'I wasn't comfortable, though,' he adds. 'I just hate all those bloody awards ceremonies. There's too many of them and they really don't mean a lot to me. It's that looking back thing again. It's not what the Clash were ever about, and it's not what I'm about.'

In September 2003, Paul Simonon made a pilgrimage of sorts to the Hebridean Isle of Raasay, where Strummer's ancestors came from. Chris Salewicz accompanied him. 'It was absolutely extraordinary,' recalls Salewicz. 'We spent days finding this derelict cottage miles from nowhere in this stunningly beautiful setting. Then Paul carted this big canvas up there and started painting. Suddenly the heavens opened, and the wind started up and his boots are so waterlogged he's taken them off and he's painting barefoot on this canvas lashed to a big stone. He was like a madman on the deck of a ship in a storm. Just incredible.'

It was, says Simonon, 'a healing moment'. He continued painting there alone for a few days. Then, when it was time to leave, he placed a Clash album inside the chimney breast of the old cottage, had a drink to toast absent friends, and rang Joe's cousin, Ian, in Texas. 'I told him where I was, in this house that Joe's great, great, great-grandfather had built. We were both brought to tears. It was quite powerful, really. That put me at ease with a lot of stuff, helped me move on.'

He grins and takes another sip of Spanish coffee. 'That's the thing about painting, it helps you work stuff out. It's a way of making sense of the world, of yourself.'

Thirty years on from the fabled Sound of the Westway, the quiet man of punk has made peace with the past, and reinvented himself against all the odds. Then again, that's what it was really all about in the first place.

· Paul Simonon - Recent Paintings is at Thomas Williams Fine Art, 22 Old Bond Street, London W1 (thomaswilliamsfineart.com) from 17 April-9 May