While at Eton, Ranulph Fiennes scaled the school hall at night and on reaching the roof scratched his initials into the stonework.

Several decades later, fellow adventurer Bear Grylls did the same, except thanks to Sir Ranulph, the authorities had erected barbed wire to negotiate as well. When he finally reached the top, he found his predecessor’s name already there.

Sir Ranulph tells me this story for two reasons: one because Bear is about to interview him on stage at the Royal Festival Hall, but also to take issue with Grylls’s claim that it was harder for him because of the wire.

“I’d have done anything for some barbed wire, something to hold on to,” ‘Ran’ declares.

But then he is the doyen of discomfort, never happy unless he’s freezing to death on an inhospitable mountain face or unconquered peak, or baking half to death while running marathons in a desert.

His achievements so far include raising £20m for charity, running seven marathons in seven days, being the first person to reach the North and South Poles by surface means only, and the first to completely cross the Antarctic on foot. And the list goes on and on. Think of your worst endurance nightmare, and he’s gleefully attempted it.

Next, Fiennes aims to walk underwater from Cape Town to Robben Island where Nelson Mandela was famously held prisoner.

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He hasn’t really done anything underwater before. It’s not really “his thing”. But he’ll “give anything a go once.”

Fiennes now has chronic back pain to add to his battle scars, caused by a pothole he tripped in during a night time marathon when his torch batteries gave out.

“Things haven’t been the same since I finished it,” he admits.

“Much worse has happened to me before though,” he says casually. “But I have such a dreadful memory these days, it’s hard to recall.

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“Though I had to take steroid injections before going up the last mountain.”

He has also chipped three frost bitten fingers off in his kitchen, undertaken a marathon just days after having a heart transplant and most recently narrowly escaped death by sleeping through a killer storm which blew away the snow walls of his base camp.

Life is nothing if not an adventure for Sir Ranulph Twisleton-Wykeham-Fiennes, 3rd Baronet – related to the Fiennes of Broughton Castle, near Banbury (including Oxford author William Fiennes) and the award-winning actors Ralph and Joseph Fiennes.

Yet despite his 75 years, he shows little signs of slowing down, finding life in polite society much more complex and complicated.

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We are here to discuss his autobiography Mad, Bad & Dangerous To Know, which he will be discussing at the Sheldonian Theatre for this year’s Oxford Literary Festival, interviewed live on stage by his publisher, Rupert Lancaster, of Hodder and Stoughton, in part to celebrate the intrepid explorers 75th birthday.

Had Ran left school with some qualifications, his life would have been a totally different experience.

After all he hails from an exemplary military family, and himself served in both the Royal Scots Greys, and the Special Air Service – where he specialised in demolitions.

Unable to think of anything else to do, he began undertaking hazardous, lethal and unthinkably dangerous expeditions, and found that not only did he have a penchant for the extreme, but that he could make a career out of it.

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The more impossible the expedition, the more sponsorship money he could raise.

“People seem to like the breaking records type things, so one does tend to do those,” he says in his extraordinarily moderate way.

And so the legend that is Sir Ranulph Fiennes began.

“I had a problem at school with maths and physics at school, and history actually. I had no languages.

I was a bit of a fish out of water actually having moved from South Africa aged 12. Luckily, I discovered that my fellow pupils liked jungle stories and they would pay to listen to them for the grand sum of a square of chocolate each.” he says with relish.

“But really I wanted to follow in my father’s footsteps. He was a decorated commander in the Second World War. That was all very well but I couldn’t go to Sandhurst unless I had A Levels.”

So eight years after he joined the army, he was forced out, unable to rise through the ranks.

“You can’t cry over spilt milk. I got married straightaway and had set off on my first major moor expedition when on leave. So from then on my wife and I planned five more together.

A prolific author, he also has 19 books to his name, chronicling his various adventures. We mere mortals live vicariously through his narrow escapes from death, his discoveries, ordeals and extraordinary triumphs.

His is certainly a life well lived.

He is a Sir, not because he has been knighted by the queen but because he inherited the title Sir Ranulph Twisleton-Wykeham-Fiennes, the 3rd Baronet of Banbury,

The nursery rhyme “” is about Lady Fiennes, she had rings on her fingers and bells in her toes to let people know she was coming, “ he informs me.

although he has received the Sultan’s Bravery Medal for service in Oman, the Polar Medal, and an OBE for human endeavour and charitable services.

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“People think I’ve spent more time in the cold than anywhere else, but actually there’s been more of the hot stuff,” he says.

And does he think his literary festival gig will be easier then his Bear Grylls debate? Perhaps Bear understands the tenacity required better than most? “You could assume that, but it’s hypothetical,” he says.

I try a different tack and ask where his grim determination and ability to endure so much discomfort comes from, along with his endless drive to overcome new challenges such as his South African mission.

“I suppose it must be in the DNA,” he says. “I’m not especially attracted to the ocean floor, I prefer mountains, but I don’t know yet. I’m just not sure we are meant to be 60ft under. I’ll let you know when I’ve done it!”

  • An Audience with Ranulph Fiennes at the Sheldonian Theatre on April 3. Part of Oxford Literary Festival.
  • oxfordliteraryfestival.org