Novelist Irvine Welsh has swapped hard-living for Miami gyms and green tea

Don't get Irvine Welsh started on Scottish independence. Actually, do. His robust, sweary, soapbox views are as provocative and entertaining as the "Choose life" speech from the film of his book Trainspotting (even though he didn't write the screenplay). We'll come to the stuff – the puff – about his highly entertaining new novel in a minute.

Resident these days in the US with his second wife, Elizabeth, the 55-year-old says American friends are always asking about the UK's political system, along the lines of: "You guys have a Queen, an unelected second chamber. You guys have a public school and Oxbridge elite running your country. How does that work?"

Irvine Welsh will tell them how that works: "It kinda works 'cause we're fucking idiots, basically. There's no reason to have any of these things. You realise that there are so many people that are grandfathered into the political system in Britain, by the time we've paid them all off, there's nothing really left for anybody else."

Press him on what he means by that, and he launches into a passionate, pub-polemic about the logic behind the end of the Union.

"This is going to be a contentious thing to say: I don't think the vote in the referendum is that important. The process is much more important. The whole thing about Scottish independence is a way of modernising the British state, and challenging it. And that's the real fear of the Establishment in England. I don't think they give a fuck about Scotland.

"If Scotland gets rid of all these hangers-on and parasites in the system – if we say, 'We're not governed any more by the House of Lords and the City of London' – then people in England are going to be thinking: 'Why do we have put up with this?' I think that's the big fear."

The breaking up of Britain, he thinks, is an acceptable price to pay for the modernisation of British politics. So if he weren't an expat, he would vote Yes come September's referendum?

"Yeah, but not in the interest of nationalism, but in the interest of anti-nationalism – against a malign British nationalism that's all about flag-waving. All kinds of nationalism sicken me. I'd much rather see people being independent and determining their own interests. And having a political system that they created that serves their interests."

Speaking of politics and policy: does the creator of some of the all-time 'great' junkie characters and rave-up/come-down narratives agree with Russell Brand that drugs should be legalised? He does, with one caveat.

"One of the only reasons I'm reticent about legalising drugs is because there's just so little for ordinary people now. If you take the criminal class, the black economy, out of the equation, there's absolutely nothing for ordinary people. The profits go to pharmacies and multinational corporations and chemists."

So he's saying he'd rather community-minded drug dealers didn't join the ranks of the unemployed? I guess it's one form of socialism.

Irvine Welsh is unwell. And late. A cold, it seems, reinforced by a two-day hangover. And, judging by his ever-busy Twitter feed, topped off with an evening hanging out at a boxing club with some old Edinburgh pals. The tardy author bustles up to the lunchtime pub behind the city's Waverley Station, all apologetic and in dire need of a livener.

But we'll let him off. Those pals, it turns out, are "the Trainspotting guys". And by that he means director Danny Boyle, producer Andrew Macdonald and scriptwriter John Hodge: the team who made the hit film of Welsh's debut book. The fantastic four have convened in Edinburgh from all corners of the globe – well, two (London and America) – for a four-day huddle.

"It's just us trying to work out how we do Porno, basically," he says of his 2002 sequel to Trainspotting. "How feasible it is... just the script really," he continues in his habitual, barely-see-his-lips-move mutter, "how it's gonna work."

So it's a summit meeting about the long-rumoured follow-up film?

"Aye, summat or nothing," he replies with a chuckle. "It's just nice for us all to be in the same room again. We're trying not to put any big pressure on us, keep expectations quite low, and just see what comes out."

Last year, Boyle told me that he would "love [Porno] to happen at some point". A sticking point had been a cooling in relations between Ewan McGregor and the Boyle/Macdonald team after the filmmakers chose Leonardo DiCaprio over the Scotsman to star in their post-Trainspotting project, The Beach. But the Oscar-winning, London 2012 Opening Ceremony director is a can-do whirlwind of the never-say-never school. Boyle seemed confident that all of the original cast would return, in the right circumstances.

"To get them back, Likely Lads-style, 20 years later, carrying that experience as people and as characters..." he said, almost wistfully. "If we can get the script right, I think they'll do it. But we'll see."

Now, a year on from that conversation, Welsh acknowledges that Hodge has written two Porno scripts "that are in pretty good shape... We're determined to give it a shot, but how far it'll go after depends on what we can produce."

Ask him if McGregor is firmly on board and he replies: "They're enthusiastic to see what we produce, but it's all about the script... And we're conscious of the fact that we don't just want to do it for the sake of doing it.

"So it's exciting. And it's just nice to be hanging out. We went to my pal's boxing club last night, and Danny and I did a round sparring with him. It's just nice to be back. It's like an old boys' reunion," Welsh smiles, then compares it to a band getting back together. "We've done a series of one-off gigs – now shall we go into the studio and try and do some recording?

"But," adds this keen student of rock'n'roll lore, "there's no point in trashing a legacy just for the sake of it. Nobody really needs the money, and nobody's really that bothered. So the idea is to come up with something really interesting and excellent about these characters that works cinematically. So we'll see."

The last time I was out in Edinburgh with Irvine Welsh, he snogged my then-girlfriend in the back of a taxi – in front of me – then attempted to do the same to Damon Albarn. I had to pay the fare, too.

But all was well that ended well: having rescued the pop star from the drunken novelist's amorous clutches, I had to undress the equally inebriated Albarn and tip him into his hotel bed. As I recall, he smelt nice, even after 12 hours on the sauce.

"Did I do that?" says Welsh, squirming a little when I remind him of his clumsy fumble with my shocked ex. "Awww..." he adds, groaning. "I'm sorry I didn't try and snog you, too. It was nothing personal."

It was late 1995. Or early 1996. Either way, the times were Britpop-tastic and Cool Britannia-brilliant. The film of Trainspotting was about to be released. Among its other winning aspects – wit, speed, energy, visual flair, realistic depictions of drug abuse, hilarity, a performance from McGregor that the actor has yet to better – Boyle's adaptation also boasted a top-notch soundtrack.

Blur were one of the bands featured. Parklife-era Albarn was in his imperial phase as chronicler of modern Britain; Welsh was, in the words of my erstwhile employer, 'style bible' The Face, the "poet laureate of the chemical generation". So, for a magazine feature, I had taken Albarn to Edinburgh to spend a day 'Touring Irvine Welsh's Edinburgh'. This meant 'Drinking Irvine Welsh's Edinburgh'.

Does the author remember that trip?

"Yeah, yeah..." replies Welsh, although he admits that the picaresque details escape him. "That whole Nineties fog basically..."

Almost 20 (ouch) years later, we're back in our former domicile. He's here in the main to promote his new novel, The Sex Lives of Siamese Twins. Set amid the body-conscious and image-obsessed of Miami, it revolves around a fitness instructor and an out-of-shape artist. The latter turns the former into a minor media celebrity by filming her muscular take-down of an armed lunatic, and their lives become toxically intertwined. His first fully American-set story, it's an eye-popping novel about counting calories, YouTube views and friends, with plenty of scenes – the lesbian sex ones – as lurid as the Miami Beach party/gym scene in which it's set.

A couple of nights previously, Welsh had launched the book with a reading event at London's Shoreditch House. American actress Brit Marling, who's in the UK to film Channel 4's cop dramedy series Babylon (Danny Boyle is executive producer), read the part of Lucy, the novel's super-fit, hardass, bisexual protagonist.

"Brit was fantastic; much better than me," Welsh grins as we settle down at lunchtime in a city centre bar, "a middle-aged Scottish guy, trying to be this young American girl. So it really worked. But then I got really smashed, for the first time in ages." As if to prove his point, that livener that Welsh ordered from the bar was nothing stronger than what he calls his preferred tipple these days: a green tea.

"In America, the culture is you don't really drink; there's not that drinking culture that we have. Maybe take your bird out for a meal, have a glass of wine, share a bottle, once in a while, but that's it. The whole thing of just going out and drinking loads till basically you're all fucked – that's a weird thing for them."

Still, old habits die hard, especially when he's back in the old country. So this week, the binge-partying lifestyle that Welsh, and many of his characters, have been known to practice, came back to bite him in the behind. "And I felt terrible for a couple of days after that do. I'm thinking: 'Why did I do this?' So I'm back into my own preferred regime now," he smiles, this big man sipping at a dainty cup.

He's flown in to the UK from Miami, via Chicago, the two American cities he currently calls home. He has, in every sense, been all over the place in the two decades since Trainspotting caused a national culture-quake. Spells in London, Amsterdam, Hamburg, Dublin, San Francisco. Eight subsequent novels. Four short story collections. Film screenplays, TV screenplays, plays. A record with Primal Scream. A video for Keane. Directing for television, travel journalism for newspapers. Rave reviews, less-than-rave reviews. And lots of famously big nights out.

But, has Welsh really, genuinely, tidied up his act? When I last saw him, talking at last year's Stoke Newington Literary Festival with John Niven (author of Kill Your Friends), both Scots authors seemed several sheets to the wind onstage. And his Twitter feed is usually a shouty commentary on this or that sporting event which he happens to be watching (football, boxing and tennis are all big faves). But he insists that his life now is more focused on health. He runs, boxes and does circuits, and is no stranger to a salad. Did this cleaned-up lifestyle, then, beget the book, or vice-versa?

"I've been going to fitness clubs for years. I was at a place called Crunch in Miami and this quite overweight wee lassie was working out with a trainer, and the trainer stared shouting at her, and she started crying. And I thought: 'She's paying for that!' And it got me thinking: what else would be in that relationship? It all came from that point."

The novel paints a dizzying picture of modern American media and politics, and of the sense-scrambling, addictive nature of physical perfection. Does he view this as a kind of collective madness gripping his adopted hometown?

"Yes, it is, with this gym obsession. But then you come back here and you think: this is madness – everyone's fucking themselves with drink! But you get immersed in it," he admits with a shrug. "You're sitting here having a few bevvies with your pals, acting the clown, and you think: 'This is brilliant, there's no other way of life!' It's quite scary to find out how environmental you actually are. There's no substance at all, you're just easily led by any place you end up!" he laughs. "You just fall into it."

Does he miss his drug days?

"Yeah," he says with a rueful air. "Everybody does. But it's not so much the drugs themselves, it's being part of this group of people who don't give a fuck and are getting wrecked and having fun. And it's also a youth thing: it's tied up psychologically with that. And everybody misses their youth."

He has to go. Messrs Boyle, Hodge and Macdonald are awaiting the resumption of their brainspotting session. Before he scarpers, I ask him: when he's writing his more extreme passages – in the new book, for example, there's one involving a fight in human faeces – does he ever wonder what his mum would make of them?

"Well, the old dear did read it," he says with a nod. "And she said, 'I like it, son. But there was too much lesbian sex'. She said I should leave lesbian sex to lesbians – 'You dinnae ken anything about lesbian sex'. And I said, 'Well, obviously I do, 'cause I've written about it!' It was," Welsh concedes, "a bizarre thing to be discussing with your mum".

'The Sex Lives of Siamese Twins' is out now, £12.99, Jonathan Cape