How does he feel about the absurd quote on the poster that says Kids "makes Trainspotting look like a 1970s episode of Blue Peter"? "In America, Miramax are using a New York Times review that said Trainspotting makes Kids look like a 1960s episode of Sesame Street,'" Welsh grins. "So I suppose that's a fair exchange."
Welsh spends a lot more time grinning than his rather granite-faced public image might lead you to expect. In fact, in person, as befits a 38-year-old enfant terrible, his mouth is perpetually twitching into a smile.
"It's really odd that I've got this kind of sullen reputation - I never saw myself that way. I think a lot of people want me to be like the characters in the books: they want that kind of congruence." What does he think it is about such warped specimens as Trainspotting's demonic Begbie or Marabou Stork Nightmares' Roy Strang that might make congruence seem an attractive option?
"I'm trying to make really flawed characters that have got redeeming features, so people can say 'I don't really like that character, but I can understand a bit where they've come from'. I like the idea of someone who is really nasty, vicious and selfish, but you can see their human side."
But people seem to respond more warmly to the ultra-violence than the mitigating circumstances. "That whole kind of voyeuristic thing is something that I find really weird. You write something for yourself and you know exactly where it stands. You're not anticipating any kind of reaction - if you do that, it's like writing with one hand behind your back.
"It's only later on that you realise that people are going to have a range of responses to things, and some of them might not be the ones that you would choose."
Welsh clearly thinks about this a fair amount. "One thing that does concern me is that young kids are passing Trainspotting around at school now. They're reading it in the same way that kids used to read Richard Allen's 'Skinhead' books, and you can't stop them doing that because kids will get into anything that they think might annoy their parents. But I sometimes think 'It's got f--- all to do with you, you little c----. Stay there, grow up, do your own thing'. [Welsh's laughter is not without a hint of anguish at this point.] Do you know what I mean?"
Welsh left school at the fag-end of the Richard Allen era to become a TV repairman - "It was that working-class, Seventies thing of getting a trade" - but jacked it in 18 months later after getting a powerful electric shock from the back of an old set ("I'll stick to hard drugs," he remembers thinking. "This TV repair game is just too dangerous.") All the people who stuck with it were on the dole soon anyway.
Welsh lived in a lot of different places and did a lot of different jobs, from ferries to property development to working for the council back home in Edinburgh. It was there that he started writing (Trainspotting was his first book), initially as a response to the horrors of the Scottish capital's late-Eighties Aids explosion.
His unflinching depiction of the worst in human behaviour has sometimes been mistaken for a lack of moral engagement. "People learn that certain ways of behaving will get them through certain situations," Welsh explains. "To others, these ways of behaving might seem inappropriate or anti-social, but that's the reality of it for them. If you don't acknowledge that reality, if you put in a moral force and say 'this is completely wrong', what you're doing is introducing the author as a character in the book, who becomes this omnipotent, God-like figure who's staging and controlling everything."
But isn't that what authors do? "It is in a way, but to me, the discipline of writing is being prepared to surrender your own ego to the force of the characters and the dynamics and the set-up. It's a psychological state, where you become a voice expressing what is happening in the culture around you, and everything I've written that I've not been happy with has been a result of failing to get into that state." Welsh admits to some dissatisfaction on this score with the first of the three stories in his new collection. Given its title (Ecstasy), its cover image (a man with very little hair holding a letter "e" between his teeth - the publishers wanted Irvine to pose in this guise himself, but he wisely demurred), and the narcotic leitmotif of minds opened by chemical intervention, this book might be regarded as a blatant attempt to cash in on his status as the literary high priest of Nineties drug culture.
Needless to say, that's not how Welsh sees it: "The dilemma that I was trying to resolve, and I don't think I have - not successfully anyway - is, if you don't have these feelings and you get them induced chemically, do you have a right to them?" That is a dangerously responsible-sounding position. "I think it's one of the great unsung dangers of Ecstasy, and it's something I'm very interested in - the psychic damage it can do to people by giving them feelings that might not necessarily be the right ones to have."
Is this a call for a drug-free society then? "I genuinely would like for there to be absolutely no drugs at all - tobacco, alcohol, Ecstasy, cannabis, whatever we need to get us into some kind of spiritual relationship. I really would like it if we could get there without needing any of that, but I think the kind of world we live in [by this, Welsh elucidates later, he means "Western consumer capitalism"] makes it very difficult for that to happen."
How does it feel as a critic of Western consumer capitalism to see people on the train reading the book of the film of your book while listening to the soundtrack album? "You've got to let go. When something gets to a certain point you've got to say 'tatty-bye' - it's everybody's now." Welsh is not about to let being the new literary establishment soften his attitude towards the old one he has always defined himself against.
"The whole breakdown of culture," he observes vengefully, "is very unsettling for those people who regard themselves as being at the cultural centre."
Why? Because there isn't a centre any more? "Because everybody's at their own centre."
It's odd in this respect that the whole Damien Hirst/Jarvis Cocker/Irvine Welsh interface, the very public index of Britain's current cultural self- confidence, tends to foster the illusion that culture all happens in one place.
"That's a slippery fish to try to grasp, but the way I look at it is that there are so many different access points, so many different routes into things now - if you watch an episode of The Bill, you'll probably get a bit of Dostoyevsky - and it doesn't matter which one you come in from. I think the great thing at the moment is the acknowledgement that things are uncertain."
It must be strange to have so much power in that situation. "It doesn't feel like that. It feels less. The minute something has a commercial base, there are all sorts of different interests involved pressurising you to do things."
Welsh has just had a falling-out with his American publishers. They wanted him to go down to London to have his picture taken by David Bailey for the New Yorker. "They ring up and say - totally straight-faced - 'You must do it because it'll really put Scottish writing on the map' and you just think 'Why? Why would you want to do that?' "
He rolls his eyes. "We've had the Brit-film invasion and the Oasis invasion, so now let's wrap everything in tartan. I'd be like [Welsh pauses to think of the worst thing possible] ... a rented Cilla Black."
5 "Ecstasy" (Jonathan Cape pounds 9.99) is out on Thursday. Welsh can be heard muttering ineffectual imprecations against the God-given superiority of the English on Primal Scream's unofficial Scottish Euro 96 anthem, "The Big Man and the Scream Team Meet The Barmy Army Uptown" (Creation single, out on 10 June).