It’s almost 30 years since former-SAS soldier Andy McNab became a bestselling author with his blockbuster Bravo Two Zero, based on his experience of being taken prisoner in the 1991 Gulf War.

"It feels really freaky," says the cheery Londoner, looking back on his career. "Until about eight or nine years ago, everything was a punt. There were no long book deals, it was literally book by book. I thought it would all stop soon.

"I only realised I was doing it for a living when I became a ‘budget’ author, when the publishers could quantify what they were going to sell."

Bravo Two Zero, the account of how he led an eight-man SAS patrol behind enemy lines in Iraq in a mission that went horribly wrong, became an instant bestseller and was adapted by the BBC with Sean Bean. It remains the biggest-selling military history book of all time.

Of the eight soldiers in the patrol, three died, four (including McNab) were captured and tortured, and one escaped. McNab, 63, says he didn’t suffer PTSD from being tortured, but later took part in a psychological study at Oxford University in 2010, which examined how people succeed in certain environments and where ‘good psychopathy’ really helps.

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The results? "I’m known as a functioning psychopath. I phoned up my wife and told her, and she said, ‘Yeah, what’s new?’

"The amygdala in your brain gives you fight or flight responses, and things such as empathy," he explains. "Basically, my amygdala doesn’t work, so I have no fight or flight response (no fear). I’ve got every decoration from the British Army, apart from the Victoria Cross, and that’s why I’ve got them."

He can’t feel empathy, either. "I can’t recognise facial expressions. People who look sad or happy all look the same to me. Years ago, my wife used to print out the emojis and say, ‘If a face looks like that [sad face], you buy flowers, but if it’s looking like that [smiling], they’re happy’. I had them in my wallet for years. People on the scale of autism have the same problem.

"At one stage, we had 12 Nutribullets still in their boxes, because every time I c**ked up, I thought I’d better buy something, and because they were the craze, I kept buying them.

"Nothing upsets me these days," he continues. "It’s quite easy for me to just cut away. There’s a bit of stoicism there, as well. If I’m in a traffic jam, I think, ‘Well, lots of people want to go somewhere’."

Reflecting on larger issues, such as Russia’s war with Ukraine, he remains similarly detached.

"It’s part of the cycle. We are going to have major conflicts, on average every 10 years. Conflicts are going on continuously, but with Ukraine, it’s on our doorstep and people look like us. Yet if you look at what’s going on in south Yemen [a civil war, which began in 2014], it’s horrendous."

(Alamy/PA)

Famously, he has always written under a pen name and keeps his identity anonymous.

"Andy McNab was just a name that would fit easily on the cover of Bravo Two Zero," he reveals. "I thought it would just be the one book, but it’s become a brand. I’ve sold beer as Andy McNab, and I don’t even drink!"

Today, he has a string of bestselling non-fiction titles and novels, children’s books, TV adaptations and an advisory role on a Hollywood blockbuster (he was technical weapons and tactics advisor on the Robert De Niro film Heat) under his belt.

His latest novel, Shadow State, the first in a new series, breaks away from his other novels, as readers enter a world of cyberweapons and crypto-crime.

The hero, Nathan Pike, is a freelance hacker who got into the game after a difficult upbringing, and now finds himself embroiled in a plot to steal a country’s fortunes on a USB stick. The action-adventure moves from the central bank of El Salvador to a tantalum mine in Rwanda.

McNab, whose cheery banter sounds about as far removed from dangerous covert ops as you can get, explains: "In the past 15 years, I’ve got involved in IT start-ups, trying to get that quick turnaround, where people have a start-up and sell it on in four or five years’ time. I started to learn the business.

"There’s just such a change now from the world of bank robbers. Why be a bank robber? If you know what you’re doing, you can steal a lot more from the comfort of your own home.

"There’s been a dramatic change in the physicality of nicking stuff. The fact is, people can buy, blackmail and steal literally from their own home. If these people were in another world, they’d be getting entrepreneurial awards, rather than going to prison," he quips, chuckling.

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There are some similarities between the new character, Nathan Pike, and his creator. Both are from working-class upbringings and had difficult lives. McNab was abandoned as a newborn baby and found on the steps of a London hospital in a Harrods carrier bag. He was fostered, and later adopted by his foster family.

Growing up on a south London council estate, he ended up committing petty crime, which landed him in juvenile detention. On release, he joined the Army, where he received an education and a flourishing career, during which he was awarded the DCM (Distinguished Conduct Medal) and Military Medal.

McNab is hoping the Nathan Pike series will continue, is planning to start his next novel in March, and would love his character to come to the screen, but isn’t holding his breath.

"I’ve had so many lunches with producers over the years, who talk about doing a film. That happens all the time. They say everything’s great, but it’s all waffle. What it boils down to, is how much it’s going to cost and who’s going to be in it."

He famously remains anonymous. You won’t see his face pictured in the media or on TV, unless he’s in the shadows or behind a book. He even says his Wikipedia entry as Steven Billy Mitchell is wrong.

"A couple of years ago, there were about seven of me on Twitter. But it’s not all cloak-and-dagger stuff."

He and his wife live in Cornwall, where he surfs, keeps in touch with a few SAS buddies, and has taken up dry stone walling, offering himself up as a labourer to friends in the building trade.

He also has a production company and is currently working on a documentary about the Royal Military Academy at Sandhurst, alongside the Ministry of Defence.

"I don’t have goals – everything’s just a punt," he insists. But he has surely learned that whatever new feat he tries, the SAS mantra ‘Who Dares Wins’ lives on.

Shadow State by Andy McNab is published by Welbeck. Available now.