The last time Sir Ranulph Fiennes tried to climb Everest, it almost killed him. So why is the great explorer planning another attempt? He talks to Jon Henley about his epic feats of endurance, sawing off the ends of his fingers - and why his adventuring days aren't over yet

The scoop is, he is going to have another bash at Everest. Which means that some time between the first week of April and the first week of June next year, a 63-year-old old Etonian with a distinctly dicky ticker and rather less than the full complement of fingers on his left hand honestly intends to be standing on top of the highest mountain on earth. In 2005, the last time he tried, he had to give up 1,000ft short of the summit ridge because "it felt as if an elephant was sitting on my chest". High time for a second shot, then. Obvious, really.

In the spring of 1979, a few months before Sir Ranulph Twisleton-Wykeham-Fiennes, 3rd baronet, set off to circumnavigate the globe along the Greenwich meridian using surface transport only (an objective which, the more geographically aware among you may realise, necessarily entails crossing the Arctic and the Antarctic continents, by land, via their respective poles), Prince Charles, the expedition's patron, announced in a moment of lucidity that the caper had his backing because it was a "mad, and suitably British, enterprise".

It occurs to me, as I successfully complete my own personal circumnavigation of Exmoor, where Fiennes lives, following a splendidly Fiennesian series of directions featuring such injunctions as "keep climbing", "ignore all turnings", "continue straight on till road peters out" and "take small track marked NO VEHICLES", that the prince's words might in fact be a fair description of the World's Greatest Living Explorer himself.

What other kind of person drags a laden sled weighing as much as three men across broken polar ice in temperatures averaging -40C for four months; runs seven marathons on seven continents in seven days; saws the tops of his fingers off when they become frostbitten; scales the north face of the Eiger on the strength of a few weeks' rock-climbing; and is determined to conquer Everest when he has already nearly died trying?

"Not a madman," he says, with every appearance of sanity. "If there is madness in there, I would like someone to specify it to me. People who undertake these kinds of expeditions have the necessary physical capabilities and clear technical skills. They have calm and stable personalities that cope under stress. They are anything but mad, because otherwise they would fail, and someone else would do it. These expeditions are very carefully planned and entirely feasible. Now if you were talking about, I don't know, hopping to the south pole on a pogo stick - that would be mad. But not what I do."

And in fact Ran, as he presents himself, really is not remotely barking. Or not so as you would notice. He is a tall, unassuming, mild-mannered, tousle-haired chap in polo shirt and much-worn jeans; of unusually athletic build, true, but sitting really perfectly calmly in an altogether charming book-lined study surrounded by black-and-white photographs of his parents, his late wife Ginny, his present wife Louise, and their young daughter (Fiennes' first child) Elizabeth. Plus some Inuits with remarkable faces.

The only clues, in fact, that he might not be, say, a rather retiring public school games teacher with a passionate interest in the history of polar exploration come from the Guinness World Records certificate on the wall ("The longest totally self-supporting polar sledge journey ever made was one of 1,350 miles in Antarctica from Gould Bay on Nov 9 1992 to Ross ice shelf on Feb 11 1993, by Sir Ranulph Fiennes and Dr Mike Stroud"), and Fiennes' desert boots, which have long, if discreet, slits cut into their sides to accommodate his peculiarly tortured feet. And, of course, that famous left hand, about which he makes no fuss at all but which, I'm ashamed to say, you really can't help noticing.

If you haven't heard the story, on the way to the north pole in 2000, Fiennes' sledge, carrying 70 days' worth of food and all his communications equipment, slipped into the sea and jammed under a slab of ice. Since he was on his own, he had no alternative but to free it, which meant removing his mitt and reaching into the water. He got the sledge out, but, as he relates in his new, updated and rip-roaringly readable autobiography, within seconds of withdrawing his left hand from the water (roughly -1C) and exposing it to the air (roughly -63C) "my fingers were ramrod stiff and ivory white. They might as well have been wood ... I had seen enough frostbite in others to realise I was in serious trouble. I had to turn back."

Evacuated by air the following day, Fiennes underwent emergency treatment in Ottawa but was told, back in the UK, that he would have to wait five months while the only partially damaged tissue healed and his "gnarled, mummified, witch-like talons" - or the top third of all his fingers, plus the top half of his thumb - could be safely amputated. Worse, he was informed, the pre-op costs were likely to be somewhere in excess of £6,000. (Feel free to skip the next paragraph if you are squeamish.)

So he decided to do the job himself. "I purchased a set of fretsaw blades at the village shop, put the little finger in my Black & Decker folding table's vice, and gently sawed through the dead skin and bone just above the live skin line," he writes. "The moment I felt pain or spotted blood, I moved further into the dead zone. I also turned the finger around several times and cut into it from different sides. This worked well, and the little finger's knuckle finally dropped off after some two hours of work." It took him five days to do the rest; a job, he says, well done.

If not madness, what's in a man that makes him do that?

Fiennes was born on March 7 1944 into a semi-noble family that arrived in England with William the Conqueror (Eustache Fiennes, according to the Bayeux tapestry, commanded the Norman army at Hastings and personally chopped off King Harold's head). A few months before his birth, his father, another Ranulph, the commanding officer of the Royal Scots Greys, was killed at Monte Cassino. Young Ranulph spent the early years of his childhood in South Africa, went to Eton where he was cruelly tormented because "it was my great misfortune to be a pretty little boy", and took a short-service commission in the same regiment as his dad in the hope of one day becoming its commander too (a feat that was never really on the cards because he never got enough O- and A-levels for Sandhurst). He was seconded into the SAS, and promptly expelled following an unfortunate incident - "that Castle Combe business" - in which he was fined £500 for plotting, with the aid of flares and some plastic explosive, to blow up bits of the set of Dr Dolittle, which was apparently causing enormous inconvenience to the good residents of that idyllic Wiltshire village. He served a few years in the army of the Sultan of Oman, and was decorated for bravery.

So exploring, he swears, was "a way of making a living. I had thought of others: I'm quite good at languages and applied for MI5 or MI6, whichever the foreign one is. A woman with a handbag and knitting asked me some questions on the steps of my Earl's Court flat. I never heard any more of it. I suspect the Castle Combe business had marked my card. Anyway, I saw Chris Bonnington [the mountaineer] making a living by finding someone to sponsor his expeditions, then talking and writing about them afterwards. His efforts were vertical rather than horizontal, but I thought the same idea could apply."

In this way, Fiennes has in the past 40 years (and in a non-exhaustive list) led a hovercraft expedition up the White Nile; explored Columbia's Headless Valley; sailed and trekked 52,000 miles along the globe's polar axis, becoming in the process the first man to reach both poles by land; travelled the farthest distance north unsupported; discovered Ptolemy's Atlantis of the Sands in the deserts of Oman; completed the first unsupported crossing of the Antarctic continent in history; run those seven marathons on seven continents in seven days shortly after undergoing a major coronary bypass following a heart attack that left him unconscious for three days; spent 72 days making it within spitting distance of the top of Everest; and, earlier this year, overcoming a lifelong sense of vertigo to scale the Eiger by a route that has killed more than 50 climbers since 1935 ("I didn't look down," he explains). And raised £10m for charity.

You don't do all that, I suggest, simply to "make a living". There has to be something more, doesn't there? Fiennes' eyes turn inscrutable; the upper lip stiffens. "I'm not much of a one for introspection or philosophy or psychology," he says. "They lead to hypotheses, which are bad. And dangerous. I can give some concrete causes and effects. For example, I can say that as a result of bullying at school I took up boxing, which was a positive step calculated to counter the pressures I was under at the time. Boxing led me to like competitive sport. Also, a deep admiration for my father, whom I never met but about whom my mother talked a great deal, channelled me into the army, and a realisation that I would never follow in his footsteps led me into what I do now."

He does not do it for the thrill of fear. In all Fiennes' epic travels, through all the countless occasions when one clumsily executed move or slight misjudgment would have meant death, he professes never to having been more afraid than when he was a young soldier in Oman. "My very worst daytime nightmares were that I would be shot and wounded in such a way that I'd be incapacitated, or made impotent," he says. "The thing is, sometimes people are out to get you. Crevasses aren't. For me, a pack of football yobbos howling on the street are infinitely more terrifying than a pack of wolves in a Canadian forest."

Nor, he promises, is he driven by a desire to emulate any particular hero, although he professes to a keen admiration for the writer-traveller Wilfred Thesiger, the late and cruelly underrated polar explorer Wally Herbert, and, rather bizarrely, our former prime minister. "Tony Blair was very popular, then he started blotting his copybook over Sierra Leone," Fiennes explains. "He sent British troops there against the will of the left of his party. It was bad news for him, bad news for the taxpayer, not good news for anyone in the UK at all - but if he stopped the slaughter and the chopping off of hands, the humane advantage to hundreds of thousands of individuals would be incalculable. He compounded it by going into Kosovo for the same reasons; and he felt Saddam posed a similar threat. If Blair's in a position to relieve or to avert suffering, he'll do it. It's deeply unjust that so many people ended up hating him."

Carping commentators have variously ascribed Fiennes' motives to masochism, aristocratic idiocy, misplaced idealism and a profoundly troubled personality. The pop psychiatrist Anthony Clare once famously decided, after spending many hours probing the innermost recesses of the Fiennesian subconscious, that there was nothing there. Analysing the explorer, he declared, was akin to "stirring a void with a teaspoon". (Fiennes, needless to say, is extremely fond of the comment and its implicit admission of failure.) All further attempts on my part to push him further about what drives him are met with a quizzical but steely look, or with lengthy and impossibly detailed descriptions of incidents from past adventures.

Fiennes' last autobiography came out 16 years ago. "Travel writing," as he calls his profession, has changed a great deal since then. Now there's Gore-Tex instead of wolfskin; GPS, not handheld compasses. "You don't even have to leave your tent to fix your position," he says. The challenges are different, and there is substantially less scope for achieving firsts, which, after all, is all that really interests an explorer. If he was a young man today, Fiennes says, he might train to chart the 90% of the world's oceans that are as yet unexplored; or look towards space.

A lot has changed in Fiennes' personal life, too. His adored first wife, Ginny, died of cancer in 2004, and he married Louise not much more than a year later. Ginny was, he says, his "wife, sister, friend, adviser, suggester, planner - my everything. I had known her since I was 12. Her death plunged me into a total, immense black hole of grief." There has since been a great deal of sniping in some sections of the press about Louise, a horse trainer more than 20 years Fiennes' junior whom he met at one of his lectures. She is popularly said to be a great deal less supportive - as if somehow that were her sacred duty - of her itchy-footed husband than was his first wife, who dreamed up most of his expeditions and accompanied him on many of them.

Fiennes is eager to defend her. "Louise is a wonderful mother, and entirely supportive," he insists. "She has not known an exploration-driven person before but she came out with me to Everest, even though she knew she would suffer from abominable migraines, which she did. She's been on two expeditions in Africa with me, and she watched my ascent of the Eiger from down in the valley - although she did feel that climb was not statistically very sensible. She thinks that to do Everest a second time would be a huge risk, and she thinks I shouldn't take it. She points out I probably would have died there the first time if I hadn't taken the pills she had given me. I've promised to be good."

By "good", Fiennes means he will take care. He is convinced that it was the pressure of climbing in - and trying to keep up with - a group that triggered his crippling angina attack last time; he reckons that if he climbs with a guide who understands that he has to take it slowly, he will get there. " The older and the more decrepit you get, the more you learn not to get too carried away. But I want to do this, and without doing anything stupid to my system." Quite why, at this stage in his life, he wants to put himself through such agony once more, he will of course not say. Perhaps - apart from such simple injunctions as "keep climbing", "ignore all turnings", and "take track marked NO VEHICLES" - there just are not the words; maybe what Fiennes does cannot be confined in language. And that might, I reckon, be why he does it.

· Mad, Bad and Dangerous to Know by Sir Ranulph Fiennes is published by Hodder & Stoughton, priced £20. He will be talking at the Congress Centre, 28 Great Russell Street, London WC2 on Wednesday October 10 at 7pm. Tickets from Foyles Bookshop, tel 0870 420 2777.