A lot of people who have lived a life of crime don’t tend to talk about it.

And, of those that do, you often wish they wouldn’t. But Howard Marks is different – funny, searingly honest and not self important.

An Oxford graduate from the Welsh valleys, Marks first tried cannabis as a student – “It was Oxford that ruined me,” he said – and went on to become a notorious smuggler.

He was moving consignments as large as 30 tons – worth £60 to £70 million – and had connections to the CIA, IRA, MI6 and the Mafia.

Chatting to him 30 years on as he prepares to bring his latest one-man show Scholar, Smuggler, Prisoner, Scribe to Kingswood House in Dulwich on May 29, it doesn’t feel like I am talking to a master criminal - or even a hugely successful businessman who had 89 phone lines, 25 registered companies and 43 aliases (including his most famous, Mr Nice).

He said: “My lifestyle then was just ridiculous.

“I think most people who make a lot of money become d***heads – flying first class and VIP and everything.

“I became a bit of a plonker, with loads of money. I don’t even hanker after money now. I feel I’ve done that.”

He added: “I would walk away from a flat carrying a couple of million dollars in suitcases, which was a lot of money in those days.

“Most of it went on consumption, a lot of it was hidden away and a lot was spent on ludicrous things – buying a mountain in Nepal, for example, which I couldn’t even find now.

“I’ve got a piece of paper which says it’s mine.”

It all came tumbling down when he was convicted in the States and jailed for 25 years. He served seven and was released in 1995.

He is upbeat about his time in prison, even though much of it was spent in solitary confinement.

He said: “I think generally - not to advise anyone at all to check into prison - it did me nothing but good.

“It made me care more about other people and stopped me taking myself seriously and realise I didn’t control events. I think emerged a far better person than the one that went in.

Since his release, as the title of his talk alludes, Marks has turned into a celebrated author and bon viveur. His autobiography Mr Nice has sold millions, and was made into film starring Rhys Ifans.

Marks is a keen campaigner for the legalisation of cannabis and stood for parliament in four seats on that platform, but these days is quieter on the issue as others take up the reins.

He is still positive about the prospects for the cause.

He said: “I am more optimistic now than I have been for years.

“I have always been too optimistic. But now, with a few states in America OKing it for medicinal reasons and Uruguay having legalised it, there is reason for optimism.”

Had cannabis been legal, you could argue that Marks would have been a respected businessman rather than a criminal.

He is more blasé.

He said: “I can’t deny I was a criminal for a long time and I made my money from crime for a long time.

“There is no point denying that. I also have to admit I still smoke dope, so I’m a criminal. I’m a criminal, definitely.”

It’s clear that Marks still gets through a lot of Mary Jane.

He said: “I smoke less than I used to but if left to my own devices, if I spend a day writing for example, I probably smoke as many as 15 to 20 joints a day.”

For the most part, he has stuck to cannabis too.

He said: “I tried them all. I never traded in them, I only traded in cannabis.

“But I’ve tried pretty much everything that exists to maintain my position as a drug expert. It’s a professional responsibility to try them all.”

Howard Marks’ Scholar, Smuggler, Prisoner, Scribe is at Kingswood House on May 29 as a fundraiser for the Crystal Palace Overground Festival from June 26 to 29. Tickets £12.50. Go to ticketline.co.uk/howard-marks or, for more about the festival, crystalpalacefestival.org