Howard Marks didn't see his latest career, as a crime fiction novelist, coming. "I didn't see any of them coming," he quips, with a throaty giggle which erupts into an alarmingly bronchial coughing fit. Since 1995, after seven years in a US prison convicted of epic international cannabis-smuggling (reduced from 25 years for good behaviour), Marks has been both a writer, whose autobiography Mr Nice sold a million, and a sort of Welsh, stoned, Billy Connolly with Bill Wyman's hair, touring his standup show of dope-crime stories like a jovial, cackling pirate. The pesky law, however, is once again threatening his livelihood, with rumoured forthcoming bans on authors profiting from previous crimes. "They'd have to ban Nelson Mandela's autobiography, Oscar Wilde's poetry, half the Bible," he muses, an unhurried 67-year-old with an air of permanent amusement. His publishers suggested crime fiction, and the enterprising Marks suggested a full trilogy, around the whodunnit dramas of Welsh Detective Sergeant Catrin Price, a onetime suicidal teenage goth. After Sympathy for the Devil (2010), now comes The Score, involving teenage murders, drug deals, betrayal and a central role for Radiohead's desolation opus Street Spirit. Marks is no fancy linguist, preferring the lure of speedy, page-turner intrigue though good reviews, so far, aren't translating to sales.

"I suppose Stieg Larsson had to write three books and die before anyone read a word of them," he laments. Nonetheless, more fiction is forthcoming. "Or, it'll be dressed up as fiction," he smiles. "If I can establish myself as a fiction writer first, then I'll write the truth." Another dramatic, chesty cackle erupts and you wonder if Marks has spent half his life with bronchitis?

"It's been a good 20% of it," he chokes. "And a small price to pay."

The first we saw Marks today he was shambling outside with a small, neat spliff, next door to Azucar, the tapas bar he part owns in his adopted Leeds hometown. It's a South American-themed, riotously decorated beanerie, and he was offered a percentage ownership in 2008, with no investment and infinite free food and drink, for the price of his public renown. "As long as I came here sometimes," he explains, now seated on a comfy black sofa. "It was a bit of a no-brainer." His nearby northern hobo pals Bez, Shaun Ryder, Kermit and Peter Hook all drink here, a Howard Marks Wall of Infamy by the bar, a collage of clippings detailing the kaleidoscopic dramas of Mr Nice, just one of his 43 aliases by the mid-80s. "The Face of a Fugitive" blares one, Marks gotcha-snapped skulking from his 70s London penthouse, alongside a 2005 photo of Marks the travel writer asleep on a coach, on a Saga holiday to Argentina, surrounded by pensioners. "Saga wanted a slightly more rock'n'roll image," he hoots. Elsewhere he's in his Oxford mortar-board finery in the late 60s, the Welsh mining village boy newly graduated as a spliff-loving (and already drug-dealing) nuclear physicist. He wouldn't have been unique in the scientific community if he had stayed in that line of work. "[Cosmologist] Carl Sagan was a smoker too!" he enthuses. "And [Nobel prize-winning theoretical physicist] Richard Feynman." Drug smuggling, he thought, would be temporary, born of idealism. "It was part of the peace and love movement so I thought: 'Obviously they're gonna legalise this shit soon rather than letting us become millionaires.'" Academia remained sidelined, and he found the smuggler's life "glamorous, thrilling", soon running 89 phone lines simultaneously, permanently on the run from the CIA, liaising with the Mafia, shipping 30 tonnes of cannabis underwater in deep-sea salvage ships and in the sky using the equipment of fictional rock bands. His greatest skill was "diplomacy", a libertine "drop-out, essentially" without a typically criminal mind, "no, because typically they resort to violence". He loved the cultural thrill of "conflicting ideologies peacefully co-existing" and the gentlemanly rules of "the shake of the hand, the look in the eye". During his incarceration in America's ruthless penitentiary, Terre Haute, Indiana, he refused to betray a single contact in exchange for early release.

"My ex-wife would say: 'Go over to the other side, come back and see your kids,'" he says. "But I wouldn't have been able to look them in the eyes. What does your dad do? 'He's a grass.' And my father wouldn't have let me back in the house. But prison isn't very high on the list of life tragedies. Far worse can happen than losing a few years as a result of losing at a game you decided to play."

Marks is a profoundly agreeable fellow, crumpled of face, eloquent of pronouncement and unassuming in his enormous sloppy cardie, packet of rolling baccy escaping from his shirt pocket. He'll smoke joints with punters here, outside, and the police never turn up. ("They've got real problems to deal with.") Cops approach him, instead, at book signings, hunting down his autograph. "A lot are open that they smoke. A lot more say they definitely didn't join the police to enforce this law." A natural raconteur, his conversation wends from Darwinism to neurology, and, of course, mind-altering substances. Unfeasibly, Marks has never been so drug (or booze) dependent he's had to give anything up; never been chained to the chimp of therapy-monitored sobriety. He has known "depressions, inabilities to cope with the drug experience, but they're vastly outnumbered by the pleasurable experiences". Post-prison, he took heroin, "to maintain my credibility as drug expert [cackles]. But the trouble with heroin is that the dose to get high is so close to the dose that kills you." He has no time for ketamine but blames himself: like cocaine, he merely took an enormous line.

"So I was unable to move," he smiles. "One of my kids said: 'Dad, you're nuts, you only take a little bit on a credit card every 20 minutes.'"

Your children are giving you drugs lessons?

"Sure! Well, they know. We shouldn't worry about giving kids wrong messages, we should listen to them."

Would a day, to you, just be boring without a spliff?

"No. It's just that I do things better when I'm stoned. Writing. Everything."

Even going for a pint of milk?

"Yes! When wankered, I'm less likely to forget the butter. It focuses the mind."

Marks has been advocating the legalisation of cannabis for 50 years. "I'm still optimistic, still patient," he decides. "That we can prove all these millions of hippies aren't talking shit." He despairs of modern politics . "There's a frightening lack of polarisation, I preferred a proper red corner and a blue corner." He sees a society lacking, generally, in compassion. "Compassion definitely needs jacking up a bit, y'know?" He ponders today's dearth of rock'n'roll outlaw figureheads and thinks technology might be to blame, "maybe you're just blocked, stopped, the carpet pulled from under you". Today he remains banned from America (where he's an "aggravated felon"), China (he money-laundered in Hong Kong) and ... Australia?

"I know!" he scoffs. "You had to have a conviction before. Now, you can't. I'd sold out in advance the Sydney comedy festival, the Melbourne literary festival, thought a visa would be a formality. They said no – I would create disorder. Then Snoop Dogg did a tour of Australia so I thought: 'If they're letting him in …' I reapplied. They said: 'Fuck off.' Then they let Cheech and Chong in ..."

Unlike most dedicated stoners, Marks has a dedicated work ethic, this year working on his fiction, essays, law reform campaigning and his on-going nationwide standup show (including Glastonbury) with the principal purpose of "getting up in the morning with lots of interesting things to do". Like a man still on the run, he spends 80% of his nights in hotels and thus, in 2013, Britain's sometime No 1 smuggling mastermind is continually busted for smoking cigarettes in British hotels. This, too, he's turned into art. "I've made a collage of all the £100 fines."

Has he always seen himself, ultimately, as a force for good?

"Global benevolence wasn't really the motive for my crimes," he says as another phlegmy chuckle rumbles forth. "But I didn't lose any sleep over it because of my frustration with the law. I've always sincerely believed that the law is wrong. Sinister. Evil, even."