Driver K44 sits in a deep armchair with his Staffordshire bull terrier, Yowsah, lying across his lap. Stroking the dog he recalls the mini-cabbing job he took in west London in the late 1980s to fund his heroin addiction. "I look like death," he tells me. "I'm driving a Talbot Solara with a dodgy starter motor and I pick up this old lady. She goes to get in the car and I say, 'Excuse me,' and I hand her a broom. While I turn the key she has to bash the solenoid. The car starts and I go, 'OK, get in. Here's the A to Z – do you know where you're going?'"

By 1989, when the mini-cabbing work became too much for him, he had taken to the London Underground, busking with a set of bongos. "Every hundred people who passed, there'd be one who'd stop and ask, 'Are you Topper Headon from The Clash?'" He shrugs, "I'd have to say, 'Yeah, this is what I do now.' It was so humiliating."

More than 20 years have passed since I last saw Nick "Topper" Headon; we were fellow drivers at a mini-cab company in Fulham, west London, where I went by the radio call-sign K42. At the time, I thought Headon had taken the state of heroin addiction to a new level; it didn't seem possible that he could drive a car at all, let alone do it for a living. When I came across his name in the press recently, I was surprised to learn that he was still alive.

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On a bright spring day, however, as I step out of Dover station, he pulls up in his customised Mini Cooper to collect me. Now 54, Headon is still small and slim. He is dressed in jeans, a striped shirt and trainers, he wears wire-rimmed glasses and his greying, spiky hair is receding. By the time we arrive back at his house, I have had a chance to adjust to how the man I knew all those years ago has been transformed in other ways, too. Where before he had the slack, cadaverous features and reduced conversational ability of the long-term heroin addict, I am now treated to his charming smile, lengthy anecdotes and frank admissions.

As if making up for his lost years, Headon is making an increasing number of appearances in the press: working with local music charities; drumming with various bands; being a spokesman for the Hepatitis C Trust, a virus that he has recently beaten. He is about to donate his beloved Mini Cooper to be raffled by the Strummerville Foundation, a charity for young musicians. To some it may seem ironic that Headon is so involved with the organisation set up in memory of The Clash's frontman, the late Joe Strummer, the man who sacked him from one of the 20th century's most revered rock'n'roll bands. Although already a heavy user, it was to be this event that propelled Headon into taking his heroin addiction to the next level.

"I just stuck needles in my arm, which I'd never done before," he says, explaining how his dismissal became his justification. "Only a junkie can think: 'I'll show you; I'll fuck myself up even more.'" More than 25 years after the event, however, the drummer has had plenty of time to reflect on where the blame really lies. "Joe wouldn't have sacked me if I hadn't been a raving heroin addict, trashing hotel rooms, throwing up, late for rehearsals." What unravels over the next few hours is the route Headon took on his downward, near terminal, journey and his recent recovery.

Topper Headon remains a hugely underrated drummer, so it comes as a surprise to learn that he arrived at the profession by accident. Aged 13, a broken leg put paid to his footballing ambitions and it was a doctor who suggested the drums as a way of venting his frustration. Within six months he was playing for a jazz band in a Dover pub.

When he later moved to London with his new wife Wendy, he was sacked from various drumming jobs for not hitting the drums hard enough, a legacy of these jazz beginnings. Drowning his sorrows at the Rainbow Theatre one night, he met The Clash's guitarist, Mick Jones, who was on the lookout for a replacement drummer.

Headon agreed to an audition but didn't bother going; he'd briefly been in Jones' previous band, the London SS, "but they were all long hair and afghans and stuff". He bought that week's edition of the NME, however, "and who's on the cover, but Mick, Joe and Paul [Simonon, bass player], and it was like... 'Oh, I'll be down in a minute, then!' I went in there and went bang! bang! bang! – I had to relearn my whole drumming style." He ended up with his hands covered in blood blisters but he'd got the job on a wage of £25 a week.

Being part of The Clash meant Headon had to give up his previous existence. Having set off for the audition in casual clothes and with long hair, he returned home dressed in punk gear, his head sporting hacked spikes. His name was changed next; Simonon rechristened him after deciding their new drummer looked like Mickey the Monkey from the children's comic, Topper. "I wondered: am I doing the right thing? I'd only been in the band a week – I'd had to deny I was married. It was quite intimidating, you had to ditch all your mates and be part of the gang."'

There was no room for Headon's marriage, but he bonded with the band through sheer industry and application: life became an endless cycle of touring and rehearsing.

It was some time before his drumming skills were fully appreciated by The Clash. His strength and stamina were obvious but his ability to play jazz, soul and funk weren't needed to begin with. Sandy Pearlman, the producer for the band's 1978 second album Give 'Em Enough Rope, was astonished by Headon, calling him "the human drum machine". "I was really on top of my game then," the musician recalls. "I didn't make mistakes. I really could drum." If Headon was gradually encouraging The Clash to play the sort of music he liked, he was also being introduced to reggae by the rest of the band. "I loved drumming, so I just thought, 'Right, I'm going to learn reggae now.' That's the way I was – I've got an addictive personality. All I ever did was drum, drum, drum. Then I went on the road and discovered booze. All I did was drink, drink, drink. Then Mick turned me onto coke and all I did was coke."

As we discuss the band's 1979 tour of the US, described by one observer as "like going on a commando raid with the Bash Street Kids", the band's former road manager, Johnny Green, arrives and happily joins in with the reminiscing.

The Clash were supported on the road by Bo Diddley. As the late rock'n'roll legend performed his soundcheck one afternoon, Headon leapt on stage and played Diddley's signature beat. For two nights, the drummer, fuelled by cocaine and alcohol, played with both The Clash and Diddley.

The two bonded immediately. As the tour bus crossed America, Strummer, Jones and Simonon would drift off to their bunks, but Headon and Diddley would be together drinking, snorting cocaine and watching the movie Behind the Green Door on the bus's video. "Which was arty porn, you know," Green tells us, straight-faced. "Thank you, Johnny," Headon replies, "I wouldn't have watched just any porn."

Headon loved being on the road but when the other band members were not performing they would retire to their hotel rooms to read or write songs alone. As their punk roots gave way to musicianship, they became more critical of Headon's behaviour. "I could see their logic," he says, "but at the same time I thought, 'Well I can't hang out with you guys because you just come back and go straight to sleep.'" Headon, by contrast, had reached the point where he was taking drugs on-stage to get through a gig. "Every three numbers I used to go... [he mimes a final drum roll] and the lights would go off and my drum roadie would be there with a mirror." Headon would snort a line of cocaine and be ready for the next song.

"The band were getting the hump with me using on stage." He couldn't see what they were getting annoyed about; he had taken drugs with all of them, too. "I'd think we were getting out of it every night," he recalls. "One night Joe would come down and we'd get drunk, the next night Paul would be down and I'd get out of it with him and the next night it would be Mick. I'd be thinking, 'This is great, we're all partying.' I wouldn't realise that only I was there constantly."

Heroin was fast becoming the drug of choice for the road crew and Headon took to it with his usual zeal. Later, he would be summoned to band meetings where his colleagues would tell him that he had to stop hanging out with the roadies. "Why?" he asked. "Because every time there's damage," Strummer told him, "the crew say, 'Well, Topper was with us...'"

Headon became so erratic and self-destructive that when they booked into hotels, Strummer, Jones and Simonon would demand to be put on different floors, but the drummer, oblivious, continued his hell-raising.

The band received an ecstatic reception when they arrived in Japan in 1982, but Headon, fresh from an enforced bout of rehab, was upset to find no drugs. He was dissuaded from his initial reaction of "No drugs, no gig" by the Japanese manager, who offered him oxygen on stage in order to give himself a regular boost. "So we get to the sound check and there's an enormous canister," he recalls, "and a nurse comes up and shows me how to use it." With his muscles re-oxygenated he was able to carry on drumming with renewed vigour.

"But the gig after that, I cut the mask off and now I've got this tube in my mouth." Headon was suddenly tackled off his stool by a Japanese stage-hand and a fight broke out between road-crew and local staff until all was explained in broken English: "Drummer... oxygen... explode!" Headon's cigarette had been dangerously near the canister. "I could've been the first drummer to explode on stage!" Headon says, on his feet acting the events out. Green nods approvingly, "That is self-destruction," he announces, "taken to a pinnacle."

By now, Headon's playing was being terminally hampered by the drugs. Strummer decided to give him a last opportunity at the next gig, which was, unfortunately for the drummer, in Amsterdam. "I don't know I'm being tested, do I? I don't know it's my last chance," he laughs, "and I'm running round trying to score coke. They're all sitting in the dressing-room, combing their hair in the mirror against the wall and I run in and go: 'Can I use the mirror?' His bandmates watched in silence as he placed the mirror on the floor and knelt beside it, chopping out generous lines of cocaine.

The drummer was "suspended" as soon as The Clash returned to London. He made an effort to pull things together but, shortly after, Strummer drunkenly told a journalist that The Clash had sacked Headon for being a junkie.

Months after being in one of the world's biggest rock'n'roll bands, Headon was living in a freezing, windowless squat in Fulham, while The Clash were performing stadium shows in the US in support of the single "Rock the Casbah", a song largely written by Headon and on which he played drums, bass and piano. He made various attempts to continue his musical career. His friendship with Pete Townshend nearly landed him the job of drumming for The Who. At the time, The Clash were supporting the British supergroup at Shea Stadium and Headon admits he would have relished playing for the headlining band, lording it over The Clash. But if drumming for The Who was ever really a possibility, Headon scuppered it in characteristic style. In full stage gear he climbed a 25ft drainpipe, ran across a roof and jumped off the other side, waking up in hospital with a broken leg and a policeman, who charged him with attempted burglary.

"In a bright red suit?" he inquired.

He formed a band with bass player Pete Farndon, recently sacked from the Pretenders for heroin addiction. "We got Rob Stoner – a heroin addict – from Bob Dylan's band and we got Pete Townshend – a heroin addict with one foot in recovery – to produce us. Then we went to Farndon's funeral – it's not funny, but within two months of forming the band the bass player's died. I mean, that was selfish – and Pete [Townshend] comes up to me and says: 'You're next.'"

Soon afterwards, Strummer sacked Jones from The Clash and the guitarist went in search of his former bandmate. He took a roadie to Fulham to "kidnap" Headon and get him back to his flat before putting him into the Priory for treatment. He was then enrolled into Jones' new band Big Audio Dynamite, and it was while they were returning from rehearsals one evening that Jones told Headon the good news: they were each about to receive £200,000 in royalties from The Clash, a sum roughly equivalent to £750,000 today.

"I went: 'See you, Mick!' Who wants to stay clean when you've got £200,000 in the bank?" The money lasted less than 18 months, at which point Headon was declared bankrupt. "My dealer used to come round and say, 'I'll take the rug,' and a big Persian rug would walk out for a gram. When I went to his I thought, 'Fuck me! This is my house!'" Headon was left sitting on the floor of his empty, remortgaged Abbey Road flat watching a black-and-white television.

In 1986 he released the largely ignored album Waking Up, in an attempt to finance his addiction. He also married his second wife, Catherine, who worked in the music industry. Six months after the album's release, however, Headon was arrested on charges of supplying heroin and received a 15-month jail sentence, which he served at Standford Hill open prison in Kent ("Horrible. But on the plus side, easy to get hold of drugs"). His musical career was effectively over: what little energy he had was used in the pursuit of heroin; when we drove together during this period he would only appear for as long as it took him to earn the £25 he needed to buy drugs.

The year 2000 saw the release of Don Letts' Clash documentary Westway to the World. Green, having long since lost touch with Headon, went to see the film, which features interviews with the band. "When Topper came on screen a gasp went up from the audience," he recalls. The driving force behind The Clash was hesitant and his speech was slurred. He weighed seven-and-a-half stone – an appearance emphasised by the oversize shirt that Jones lent him when Headon arrived for the interview in a T-shirt full of cigarette burns.

With his second marriage behind him, Headon found himself in St Mungo's hostel for the homeless just yards from the Westway, the elevated west London road The Clash had claimed as part of their urban identity, living on cans of Special Brew supplemented by twice-daily visits to soup kitchens.

He pauses. He's been trying to make light of this time, but as his story has got darker so has the mood in the room. Until this moment, I have had the feeling that Headon is finding it cathartic telling his story, but this is a period of his life that, even now, he is having difficulty coming to terms with. "I've been laughing," he tells us, shaking his head, "but this was..." he stops, searching for the right word but failing to find anything that goes far enough, "...horrendous."

He returned to Dover, becoming the local drunk, cornering people in pubs and shouting at cars in the street until his doctor told him that his liver was "waving the white flag". Some time during the 1980s Headon had – not uncommonly among intravenous drug users – contracted hepatitis C, which can cause cirrhosis: "It was the least of my worries," Headon says. He returned to heroin, before his doctor stepped in again, convincing him to move in with his parents before going back into the Priory. He has been through rehab 13 times, but this time it worked. Why? "I don't know. Something happened. I started feeling part of life again. I've been clean ever since."

He launched a Narcotics Anonymous group in Dover and set up a hepatitis C support group. Last year, he briefly appeared on stage with Mick Jones' band Carbon:Silicon for a couple of Clash numbers.

As for the break up of The Clash, he knows that Strummer was unhappy with the band The Clash had become; playing stadium concerts was the antithesis of Strummer's view of what The Clash should be. "Joe was sacking everyone else rather than just leave himself," says Headon, who bears Strummer no malice for the manner in which he was dismissed. "He had no choice," he explains. "I was in a state. We were kids," he shrugs. "Who cares?"

Strummer certainly did. He blamed himself for The Clash's demise, admitting that the band "never played a good gig after Topper left". With hindsight, however, Headon is glad it ended when it did. "It was the best thing that could have happened. We made all that fantastic music and then imploded at the top." He concedes, however, that what was best for the band was not necessarily best for the band members.

In 2002, Strummer made the only serious attempt to reform The Clash, to play at their induction into the Rock and Roll Hall of Fame the following year. Jones and Headon were ready, Simonon was against the idea "but Joe had lined up another bass player. And then he died."

Today, Headon feels that he is finally over the five years he spent in The Clash. He has borne the brunt of the blame for the band's break up, compounding the problem by making the widely misunderstood statement in Westway to the World that if he had his time over, he would do it all again. "I was trying to explain that when you're a heroin addict you don't choose to fuck your life up, it's inevitable." At one point, as we discuss Strummer, Headon interrupts. "By rights," he says with conviction, "it should have been me that died."

In fact, the drummer looks remarkably well. He keeps fit and his kitchen cabinets are crammed with health supplements. When it is time to leave he drives me to the station, stopping en route at Dover harbour where we watch the ferries heading out to France. "I'm just a middle-aged man by the sea," he muses. Later, I repeat this remark to Green. The Clash's former road manager is dismissive. "Rock stars are like Catholics," he tells me. "They may lapse, but they're still rock stars."

Topper Headon's Mini Cooper is being raffled online at www.strummerville.com with all proceeds going to charity. The winning ticket will be drawn on 13 July