Irvine Welsh: Upwardly mobile

Irvine Welsh, the creator of the charismatic junkies in Trainspotting, has revisited them for a new book, with added porn. Disreputable characters have made his name, but now Welsh moves in more elevated circles. He's even been mistaken for a famously middle-class author...

Poor Irvine Welsh. He can't go anywhere these days without being monstered by his adoring public, even on his adopted turf of north London. "I was in Waterstone's the other day," he confesses in his sotto voce mutter, "and this woman came up to me and said, 'It's great to see you again.' And of course, you meet so many people at readings in bookshops, I just said, 'Hi.' She was being really complimentary about my books, and I was going, 'Yeah, yeah.' Then she said, 'I do think How to Be Good is the best book I've ever read.' I didn't know what to reply. Eventually I said, 'It was a difficult one to write, but I'm glad I persevered.' She said, 'I'm glad you did too.'"

Irvine Welsh Mistaken For Nick Hornby Shock! On the face of it, one is surprised it doesn't happen more often: both men are famous novelists, both are passionately keen on football and music, both live in the immediate environs of Highbury and both are radical slapheads. On a more subtly cultural level, however, the Welsh-Hornby mix-up is very significant. Meeting the working-class Scottish guru of the E generation, you're aware of meeting a man on the cusp of respectability, flirting (or is it wrestling?) with the magnetic pull of becoming as middle class as Hornby's troubled bourgeois.

It's all due to his becoming a huge literary success, a gob-smackingly global cultural phenomenon, and enjoying the benefits that go with it. Trainspotting was published in 1993. It sold millions, and the movie version, directed by Danny Boyle, became one of the Zeitgeist-defining films of the Nineties. The Leith dialect of Renton, Sick Boy, Spud, Begbie and their wasted, disreputable pals was translated into a score of tongues in countries you wouldn't associate with Scottish junkie culture and pub humour – Lithuanian, Korean, Hebrew, Arabic. Welsh is constantly surprised by his own fan constituency. Until he went to Bulgaria, he'd no idea he was regarded as a kind of rock star. "I couldn't believe it. It was like Beatlemania. I got off the plane and the Bulgarian paparazzi were out in force. I was ushered into the party members' special enclosure, then given a limo to the hotel. I got mobbed. I couldn't go anywhere without drawing crowds."

The global phenomenon blinks at the recollection. He seems mildly bouleversé by the world's enthusiastic embrace, but only mildly. A Zen-like calm comes into his pale green eyes as they survey the vanity of middle-class complacency in London N1. He has no intention of joining it, but, hell, a few concessions can be made when you've sold millions of copies of the rotting fruits of your imagination. "I admit I do indulge myself sometimes. I do travel first class occasionally. I go to better restaurants than I used to." He didn't, I observed, go for Savile Row suits just yet – he's wearing jeans, a nondescript green T-shirt and a horrible stripy nylon Adidas jacket. "Not today," he says, "since I'm havin' my picture taken." He doesn't own a car and never has, which is a shame since cars are such eloquent social signifiers. "But maybe I should," he says eagerly. "Maybe it's time I bought a big, fuck-off sports car..." And he sits, this potholer of the lower depths, this school-of-Keith-Richards connoisseur of druggy excess, this mad survivor of the five-day binge, he sits sipping mineral water in a café in Islington's Upper Street, the golden mile of New Labour triumphalism, discussing his new exercise regimen and his success at running the London Marathon twice.

What lies behind all this fencing is Irvine Welsh's reputation as a writer of rare authenticity – as the genuine voice of the Edinburgh "schemes" (or council estates), the street idiolect of Leith Walk junkies, pimps, psychos, bampots, jakeys, gadges, skagheads and their less attractive peers, as they wander through a Bosch landscape trying to score money, drugs and sex. Some readers took Welsh's books as unmediated reports from the frontline of screwed-up youth, failing to notice the acute literary intelligence that worked magical feats on plots, multiple narratives and airy cultural allusions.

He followed up Trainspotting with more grotesque street tales in The Acid House, then the astonishing Marabou Stork Nightmares, which twinned extreme male violence with an idiom of Biggles-and-Kipling Imperial braying. It was praised by John Carey, the top Oxford Eng Lit luminary.

But as Welsh moved into his one-word-title phase with Ecstasy, Filth and Glue, you could hear his reputation suffer an audible pressure drop. It's just more of the same effing and blinding and dancing in the cesspit, the critics said, tricked out with special effects (like the tapeworm that chews its way across the rancid confessions of Bruce Robertson, the corrupt cop in Filth). In Glue, figures from earlier books began to reappear. Was he starting to repeat his effects? Was he running out of ideas?

Heaven knows what his critics will say about Porno, which is a full-on, shameless return to Trainspotting, and reunites the four main characters. Why do it at all? "I always liked them, and I always wanted to go back and see what happened to them. But it took me a while to really want to do it, because you've got to make it feel like it's still them, but 10 years further on. It's got to stand up to the test of the original, you know?"

Trainspotting ended with Renton walking out on his mates in London – Sick Boy, Spud and Begbie – stealing the drug-deal loot and embarking on a life away from the Junkie Hell of Leith. When Porno opens, we find them in various states of disarray. Sick Boy is working in a London strip club, trying to become a pimp. Begbie is in a Scottish jail, awaiting release. Renton is lying low in Amsterdam. Spud, the hopeless loser, has acquired a girlfriend and baby and is planning to write a history of Leith. They are like the components of a time bomb waiting to go off. Then Sick Boy takes over his aunt's scabby Leith pub, which becomes the convergence point of the warring factions, while the upstairs bar is the site of increasingly disturbing excursions into what Sick Boy calls "the narrative of pornography: our sequential journey". A cast of new characters fleshes out the novel's titular fascination, especially Nikk Fuller-Smith, a sexually voracious student of film criticism who works in a massage parlour to earn money and finds her métier in celluloid orgies.

There's a curious joy in re-encountering the psychopathic Begbie, a man who puts the frighteners on the reader along with everyone he meets. But the real revelation of the new book is Sick Boy, who has mutated into a full-on style hero, real name Simon David Williamson, a swaggering, Armani-suited, endlessly inventive, breezily manipulative con man, sexual athlete and scam artiste. His narrating voice moves easily between elegant put-downs and seduction routines to down-home vulgarity and moral blankness. He has the best lines. It is, sometimes, a bit of a relief to encounter Sick Boy's suave ironies after pages of Spud and Begbie's thick-ear demotic ("Ah looks oot the windae n sees thum gaun doon the road..."). Was there any likelihood that a future Welsh novel might go for the elegant stuff at the expense of the street lingo? Could he ever write a novel full of middle-class voices? What would it be like? "Like a novel by Nick Hornby," says Welsh. "But he does it better than anybody else. And there are many people who do this kind of thing well, so why should I bother?"

Pornography isn't just a metaphor for the cheapening of human relations in late-Nineties Western society. As far as Welsh can see, it's all around. "Ten years ago," he says, "drugs were part of youth culture, and now they've gone mainstream and consumerist and they aren't part of our culture any more. I think porn's going the same way. The only thing stopping it is some old legislation. Everyone wanted to see people shagging on Big Brother, but it wasn't allowed because of TV legislation. But the internet and digital video have changed the whole face of porn. It's everywhere now. Home-made porn is everywhere. Shag clubs have set up everywhere – there are loads in Edinburgh – and it's a whole new generation getting into these clubs and into porn. It's easy to get a knocked-off DV camera in a pub, and shoot your own stuff and put it on the Net. It's become a massive operation. And because it's been taken away from the back streets, you get more women consumers involved, it's become a mainstream thing." He pauses to draw breath. "The film industry used to be separate from the porn trade, like on parallel lines. Now they're crossing over. Now you get Greg Dark making Britney Spears videos. Times gone by, they'd have said, 'Fuck me, no, he's a porn director.' Now they say, 'Oh, he's just the right person to make the video.'"

He began looking into porn while living in Amsterdam, where he chilled out for a few years after Trainspotting went supernova. "Yeah, I did some research into the movies. But I've never bin a big consumer of porn myself. I've never really got off on watching other people shagging, I've sat there thinking, I should be up there." He is ambivalent about the sexualisation of society. "There used to be a place for porn," he says, "but now its place is everywhere. That's the problem with modern consumer capitalism, it goes for the lowest common denominator – flog the drugs, flog the porn, because that's what people want, or can be made to want very quickly."

This offhand pop at capitalism is characteristic. Welsh often slips into O-level agitprop; he has a virtual fetish about the word "consumer", to go with his fear of turning into a bourgeois breadhead. Like Sick Boy, he is from the "lad o'pairts" tradition of Scottish autodidacticism, as celebrated by Robert Burns, the "Heaven-taught ploughman". Welsh's father was a Leith docker, his mother a waitress. He left school at 16, got a job as a TV repairman but jacked it in and headed for London just as punk was breaking out. He hung out with "bedroom bands" but without success – he has always been a frustrated club DJ but lacks, he says, the expertise – and turned to property management, studied for an MBA, became training officer for Edinburgh District Council; and it was then, while bored with training, that he began to excavate his recent past for Trainspotting.

He never expected what happened to happen. "I thought it might be a big kinda cult book in Scotland, but I didn't expect it to go outside at all..." Never a man to bandy literary influences or allusions around (though his work is full of the latter) he is very well read in postwar Scottish literature. "There was always a big Scottish book every coupla years when I was growing up," he says with what could almost be national pride. "First was William McIlvanney's Docker, then James Kelman's The Bus Conductor Hines, then Alasdair Gray's Lanark, then Iain Banks's The Wasp Factory, then Janice Galloway's The Trick is to Keep Breathing, er, then Alan Warner's Morvern Callar..." Between Galloway and Warner in this gallery there is one huge and obvious gap, where Trainspotting should be. But Mr Welsh is far too polite, too well-mannered these days to blow his own trumpet.

He has had a Variety of Projects (as they say in the Groucho Club) on the go in recent months. He produced a play with songs called Blackpool, performed by students at Queen Mary's College, Edinburgh. Why? "I just fancied doing a British musical and thought Blackpool was a great subject. I went there for a stag night and it was a bit like going to Las Vegas. Two nights you could just about handle, but three is too many. I was there for four. It was just so British. You get parents worrying about their kids going off to Ibiza, and being corrupted by Johnny foreigner. They ought to see what happens in Blackpool. I saw some amazing scenes – there was a guy shagging his girlfriend in a doorway, doing it from behind with his arm round her hips, while she was eating her way through a bag of chips." Welsh briefly mimed the lady's bovine, unhurried eating technique. You could see her taking up residence in his head, waiting to be downloaded in a novel.

He's finished co-writing a screenplay called Hotel California, to be filmed for BBC television by Antonia Bird. He doesn't know who's going to be in it but describes it as "Midnight Express meets Romancing the Stone". But his immediate worries are about the launch of Porno this week, the party at which his multifarious acquaintance of criminals, bricklayers and arty types will inspect each other with mutual suspicion over the canapés, and the can of worms he may have opened by writing a sequel at all.

Months of razzmatazz, transatlantic touring, signings, readings, random street monsterings and Nick-Hornby-sightings will make up his life for the next few months. The film world will kick in ("There's bound to be a movie," he says, almost gloomily. "There's far too much money involved for there not to be.") and the several industries he feeds will begin tapping their fingers in a year's time, waiting for the next milch cow to be delivered.

"You know what I'm really afraid of?" he asks. "Sick Boy becoming a franchise. I worry that I may be turning him into a schemie, coke-fuelled Harry Potter." Goodness. Stand by for Sick Boy and the Goblet of Vomit around 2005...

Deborah Ross is away