[In the following interview, Welsh discusses his double past as junkie and business student, his political leanings, and his odd position of straddling fringe and mainstream culture.]

Entertaining Irvine Welsh is a nervy business. For a start, it is hard to know quite whom to expect. The penultimate sighting of Welsh was in Elle magazine, where he was pictured at the sparkly launch party for his (much-reviled) play, You’ll Have Had Your Hole, surrounded by such literary heavyweights as Mel G and All Saints. Two days later, the author of Trainspotting was seen in a railway carriage in the advanced stages of a protracted bender and—following passenger complaints about his behaviour—arrested and reportedly hauled off to the police cells for five hours. On recent evidence it seems wisest to hide all naff CDs (Welsh, Spice Girl connections notwithstanding, is a serious musician) and get in the beer.

Neither precaution is necessary. He arrives promptly, requests sugarless tea and admires the back garden. He is as non-confrontational as a gas meter reader and almost as anonymous; a big, amiable bloke with a razor-scraped head and bluebottle-green bovver boots. I would not have recognised him, but he says he is always being singled out by pub drinkers seeking literary chit-chat or, sometimes, by police who vaguely recall his face and assume it belongs on a soccer thug register. He loathes intrusion, shuns publicity and has agreed to drop round for a rare interview partly because he is keen to talk about the Scottish elections.

Not that he will be voting. “I can't, because I’m in London. Anyway, I can't remember the last time I voted in any election. People in Scotland want the parliament but don’t give a toss about the elections. They think they will reward Labour for delivering. For the SNP it's a no-lose situation. If it doesn’t work out, they can say it's a tied parliament. If it does, they can say that Scotland has shown its expertise in governing its affairs.”

Although not a Salmond man (“I couldn’t vote for him: he's a Hearts supporter”), Welsh (Hibs) sees himself as a post-nationalist, convinced that British identity is “in terminal decline” and that the ultimate future for Scotland is full independence. In the shorter term, he is buoyed up by the hope of more cultural vibrancy but still moderately pessimistic. “The worst-case scenario is that the parliament will be crap … You always get an oligarchy that creams it off for themselves. There is such a moribund infrastructure of deadbeats and conmen in the Labour Party that has dominated politics in central Scotland for so long. I’d be absolutely astonished if these people didn’t manage to push their noses in the trough and dominate. Hopefully not. But as someone who has worked for local authorities in Scotland, you always live in fear that will happen.”

The Prufrock phase of Welsh's life began when he became a clerk in the Edinburgh council housing department; a job he took after he had weaned himself off heroin. In the past, Welsh has been cagey about his time as a drug user, but he is eager now to trawl through his past. He was born in 1958 in Leith, the son of a docker and a waitress, and grew up in the thin prefabs of Muirhouse—a soulless dumping ground for tenement families. In his account he first appeared in court at the age of eight, for playing football in the street, and he was a proto-junkie who “bought Airfix kits, threw away the plane and inhaled the glue”.

After leaving school underqualified and almost innumerate, he drifted between Scotland and London—working as a TV repairman, sleeping rough, playing the guitar and, in an Orwellian interlude, washing dishes in a restaurant. Though he does not put it so, Welsh also hedged his bets. He gradually acquired a portfolio of useful qualifications—from a City and Guilds certificate in electrical engineering to an MBA from Heriot-Watt. In addition, he majored in crime and drugs. While he paints this spell as a typical skid row, there is some sense that his odyssey—a career of shoplifting and break-ins fuelled by cannabis, LSD, amphetamines and heroin—was less a nemesis than a research programme.

Certainly his three years on heroin do not seem to have brought him to the verge of ruin. “What stopped me getting really bad was having crap veins,” he says, but you suspect there was some other factor—a rigid self-control that allowed him, now as then, to veer between wild binges and asceticism, between self-destruction and self-interest. He implies some unease about his credentials to write Trainspotting, which was admired by those cosily distanced from the drugs underworld but regarded with mistrust by some who recognised their own misery. “I’ve known people who’ve been junkies for 25 years, and they say: ‘How could you write this book? You were only on smack for five minutes.’ I felt I’d been on it for long enough.”

Other things make Welsh defensive. He loathes being portrayed as a Rousseauesque noble savage—“a thick fucker from a council estate”—but he is also scornful of his rating, in an Observer/Channel 4 pre-election survey, as the 43rd most powerful man in Scotland. “There's the need to convince people that this is some kind of pluralistic society, so they string a few writers read by younger or more working-class people in with the mercantile classes.”

The irony is that many in the golfing and business nexus would envy Welsh his new affluence. His latest novel, Filth—about a corrupt, sex-crazed, racist, woman-hating detective—sold 250,000 copies in six months and Miramax has bought the film rights. A film of his novel The Acid House has just been released in Australia. Ecstasy, a trio of novellas, is out in Europe, and Welsh—hugely prolific—is now billed as a multi-millionaire. “You’re joking,” he says, faintly embarrassed by his wealth. “I see reports of how loaded I am, but it's only in the past two years that I’ve started to make any serious money. There was this big fuss when I bought a flat in the New Town in Edinburgh. It was £120,000. You don’t buy that if you’re a millionaire.”

Welsh splits his life between Edinburgh and a second flat in Stoke Newington, north London, whose anonymity suits him. He is fantastically prim about his private life, refusing to confirm even that he is married, and phobically nervous of the press (“I won’t speak to tabloids”). A despiser of small talk, Welsh is expert at large talk. He writes, he claims, in Edinburgh vernacular chiefly because he can't spell, and his conversation—a shambles of “sortalikes” and “kindalikes”—veers between stream-of-consciousness rambles and nuggets of pith. “There's fuck all to say about my books other than what's written in them,” he once beerily told the Sydney Writers’ Festival, weary of being branded the authentic voice of the chemical generation and the dispossessed.

Now an inevitable backlash has begun, and Welsh—funny, dangerous and once almost universally acclaimed—is reviewed in cooler terms. His play, which closed after a fortnight when the lead actor slipped two discs, had particularly terrible notices. Welsh is sanguine. “It's easier, in a way, for me to deal with rejection than success. If you can't be liked by really cool punters, the next best thing is to be hated by arseholes.” But Welsh—sometimes more disturbed than his critics by his subject matter—is not always so flip.

“My work is very bleak. I compartmentalise. I’ve been going through a work ethic phase, which is why I got absolutely smashed after the play. It's difficult to switch off. If you’re writing about racist, misogynistic characters in the first person, you do get quite fucked up by it.”

Others also get confused. In the run-up to the elections one Edinburgh paper wrongly claimed that he was funding the Scottish Socialist Party candidate and a week later implied, entirely erroneously, links with Combat 18. “That's a kind of class thing. People think if you’re working class, there has to be some fascist element underneath.” In fact he is well to the left of Tony Blair (“Isn’t everyone?”) and abhors the strident English nationalism emerging as a by-product of the Scottish elections. “People should be able to express their culture without getting into all that chauvinistic thing. It's ironic that the growth of Scottish nationalism has precipitated in the English the sort of hand-wringing the Scots have always done over who they are.”

He now foresees a wholly independent Scotland. “There's no reason not to. You get all these nonsensical arguments that it would be either absolutely impoverished or like a rich Scandinavian country. It can be what it wants. In terms of assets, no one can say it will be poor. It can be mismanaged, but so can any country.”

Welsh loves Scotland, returning frequently for his fix of clubs and football. He also acknowledges that, in terms of work, his Edinburgh past is an exhausted vein. He is unimpressed by the short stories he is now trying to write. “To be honest, they are crap. I think I’m going to throw them away. I’ve exhausted any autobiographical content, and there isn’t anything socially significant in my life now.”

He calls himself an internationalist, but the term seems to reflect not so much a wish to be a freewheeling member of the Scottish diaspora as a need to escape from a staid writer's life by recreating some vestige of the now elusive past that fuelled his best work.

“I was in Amsterdam with Howard Marks last weekend, and I had a whale of a time. There's no way I could be so uninhibited here. I have to be so much more restrained,” he laments.

It is tempting to ask Irvine Welsh—charming, tractable and politely sipping tea—exactly what kicks he finds in his wilder excursions. I would guess they are part research, part reassurance, evidence that Welsh, who has so brilliantly exploited his roots, is conscious also of the downside. Any working-class Scot who finds himself in the society columns of Elle must worry less about media prurience than the risk that he is writing himself out of his own plot.