The ex-SAS officer had led private forces in Angola and Sierra Leone but then, hours before he was to launch a coup in Equatorial Guinea, a tiny oil-rich country in west Africa, everything went horribly wrong. In his new autobiography, Cry Havoc, Mann tells how he was intercepted by Zimbabwe’s secret police and thrown into Chikurubi maximum security prison. Three years later, he was extradited to Equatorial Guinea, where he was sentenced to 34 years in the notorious Black Beach jail. His ordeal ended in November 2009, when, after much wrangling behind the scenes but for reasons that remain opaque, he was pardoned and released.

“The most frightening was when I got kidnapped from Zimbabwe and taken to EG – I remember thinking that I would certainly be shot on arrival,” Mann says. “And then, in EG, I was tried again, sentenced and thrown into solitary confinement. I remember wondering, ‘maybe they’ll keep me here like this for the next 30 years’. It wasn’t good.”

Mann’s crime was to plan, with the clandestine support of some foreign governments and international oil interests, the overthrow of Teodoro Obiang, the dictatorial president of Equatorial Guinea. It was to be, in Mann’s words, ‘Wham Bang, Thank You Obiang’.

“We land at night, with a crack unit. Our weapons are speed and surprise. We smash and grab the palace, with Obiang inside, then Army HQ, Police HQ, communications centres, banks, media centres,” Mann writes of his plan. “We seize power in a lightning strike. We mug Obiang of his personal fiefdom. A blitzkrieg smash and grab.”

Although Mann’s coup was rumbled at the last minute, while his troops sat, waiting to fly into battle, on the runway in Harare, he insists his only regret was getting caught.

“If faced with all the same factors, I’d end up doing it all the same again. Obviously, if I knew it was going to fail, I wouldn’t go through with it,” he says. “I had cold feet. I knew things were wrong. But there was so much time and willpower invested. I suppose it’s a bit like serious mountaineering – you don’t succeed by quitting easily.

“I had signals that the South African and Spanish governments both wanted it to go ahead, almost telling us to get on with it. And it was also my experience in Angola and Sierra Leone, where we’d been in weird situations before, but by pushing on and being excessively ballsy, we’d won. I thought we could do it again.”

Mann’s motivations for the ill-fated coup were complex but he maintains it was, fundamentally, a humanitarian mission.

“If I hadn’t thought Obiang was a rank tyrant, I wouldn’t have done it,” he says. “I was invited to do it by the leader of the opposition, Severo Moto – he was chucked into prison after being elected mayor and later exiled. If you say, ‘Fuck off, I don’t care’, I don’t think that’s right.

“I didn’t need the money – I had plenty of money already. But after spending so much time in Africa, I felt quite strongly about all this crap government. There are millions of people whose lives are ruined because they’re governed by arseholes. I thought, ‘We can do this – why not?’”

Mann’s detractors, however, suggest his real objectives were less philanthropic, that he was driven by cold-eyed capitalism; as a mercenary, Mann was taking his skills to market, hoping to get paid. Certainly, Mann is not disingenuous enough to argue that money wasn’t part of the pay-off – instead, he writes openly about the spoils of war, about the “supertanker-loads of petrodollars to be made”.

“If the Brothers-in-Arms put Moto in power, Moto will see to it that the Brothers-in-Arms benefit from EG’s great wealth,” Mann writes. “Quid pro quo.”

But, perversely, for Mann, the thrill was seemingly as much an incentive as the financial reward: “My thinking was that we could bring him down and also make a lot of money. There was also an adrenaline aspect – it was a mountain that needed climbing. And I was flattered – I had been retired but these people came to me and said, ‘You’re our man’. So yeah, I wanted to make a lot of money and I wanted another adventure. In the end, I thought it was worth doing.

“People might think, ‘Who the fuck are you, Simon Mann, to go charging around Africa doing this?’ But after seeing what a basket case the country was and seeing lives ruined by a criminal government – what’s the difference morally between going to help them and stopping someone being mugged? What if there are no police?”

Surviving jail in Zimbabwe and then Equatorial Guinea required, Mann says, “a balancing act between hope and despair”. But then, five-and-a-half years after first being taken prisoner, he was released, almost out of the blue, for reasons he still does not fully understand.

“When they told me, I didn’t believe them, I went into a kind of shock,” he says. “I still thought it wouldn’t happen, because so many things had gone wrong, but then I realised it must be true and after that it all happened very quickly.

“There are many aspects about it where I simply don’t know and I don’t think I’ll ever find out. Who knows? And are those people ever going to tell me? It’s a shady world where nothing is quite as it seems.”

It is a world Mann seems happy to have left behind. Away from the theatres of war, he hopes to forge a career as a novelist and there is already a film planned about his misadventure, with Gerard Butler pencilled cast in as the lead.

“I’d quite like to get into writing some fiction and there’s a film that’s looking quite likely,” Mann says. “We’ve done the deal. Gerard Butler is a good guy – I’ve met him a few times and I’m very happy he’s attached.

“But, no, there are no more coups planned. All those tyrants can relax.”