Were he 33 now instead of 83, he is not sure that he would even bother with tackling the famous peak. "There wouldn't be any of the feeling of achievement. It's been dealt with. So much is known about it." He pauses. "I suppose I would want to climb it," he smiles.

He talks about his experiences with the bluff modesty of a Boys' Own adventure hero. When he set off for the Himalayas, he was a "reasonably good climber". Of his achievement, he concedes that he and Tenzing "did a pretty good job".

On reaching the summit of Everest, he initially went to shake hands manfully with Tenzing before the Sherpa threw his arms around him and slapped him on the back. "I wasn't extremely excited," he says. "I didn't jump around, but I had a pretty strong feeling of satisfaction. It was a very good moment in that sense."

It seems almost underwhelming to be greeted with this matter-of-fact attitude from arguably the most famous mountaineer of all time. The most serious climbers are popularly believed to have an almost mystical dedication to the peaks they attempt. Reinhold Messner, the Italian who became the first to scale Everest without oxygen in 1978, described a "state of spiritual abstraction" at the summit. "I no longer belong to myself and to my eyesight," he wrote. "I am nothing more than a single narrow gasping lung, floating over the mists and the summits."

Even John Hunt, the stiff British army colonel who led the 1953 expedition, refused to talk about the "conquest" of Everest, only the "ascent" - a concession to Tibetan reverence for Chomolungma, the goddess mother of the earth, that carries a whiff of superstition about it. Hillary's down-to-earth attitude is the exception to this rule. "When you go to the mountains you see them and you admire them. In a sense they give you a challenge, and you try to express that challenge by climbing them," he says.

It is a long way from Mallory's own sonorous metaphor. "If you cannot understand that there is something in man which responds to the challenge of this mountain and goes out to meet it," he wrote, "that the struggle is the struggle of life itself upward and forever upward, then you won't see why we go. What we get from this adventure is just sheer joy. And joy is, after all, the end of life."

Hillary's response to reaching the summit was more succinct. "Well, we knocked the bastard off," he told his companions on returning to base camp.

I ask him if standing on the peak of the highest mountain in the world felt qualitatively different to conquering another, equally difficult peak, but he bats it away. "Oh, I definitely knew I was on top of Everest," he says.

He seems impatient of too much self-analysis, disliking all the symbol and metaphor that has accreted to the mountain. But for all this self-effacement, in 1953 Hillary found himself caught up in a stage-managed moment of high patriotism. No matter that the pioneers had been a New Zealander and a Nepalese, the expedition was British: in the photograph taken at the summit, the British, Nepalese, Indian and United Nations flags flutter, but Hillary's native country goes unrepresented.

Amid the dying remnants of the empire, the news of this final grand exploit acted like a tonic. The first reports of the ascent arrived in London to coincide with the Queen's coronation. A week later, Hillary and Hunt were knighted (Tenzing missed out on this honour, because he was not a British citizen).

It was no accident that the triumph coincided so neatly with the coronation. The Himalayan Committee which sponsored the ascent was an avowedly nationalistic body. Eric Shipton, the great mountaineer who had been Hillary's inspiration and mentor, was thrown off the expedition in favour of Hunt because of fears that he would not push the Nepalese authorities hard enough to start the climb in time. The French were booked for 1954, the Swiss for 1955, and, as a favourite mountaineer's dictum has it, "No one remembers who climbed mount Everest the second time."

Their success sparked a brief golden age of mountaineering. Of the 14 mountains whose peaks rise above 8,000m, only Annapurna had been scaled when Hillary and Tenzing stood on top of Everest. Just five weeks later, the world's ninth highest peak, Nanga Parbat, was topped, and by the end of the decade only Shishapangma, which lay off-limits to western alpinists in Chinese Tibet, remained unconquered.

Since the early 1970s, the trickle of adventurers making it to Everest's summit has become a flood: 1,500 pairs of boots have trekked their way to the top, ropes and tents lie abandoned in the snow, and glacial crevasses are bridged by aluminium ladders. Two years ago the Nepalese government began a clean-up operation on the mountain after nearly 100 tons of rubbish was discovered on its slopes.

Hillary's son, Peter, has climbed the peak twice, and three generations of Tenzings have stood at 29,028ft. There have been husband-and-wife ascents, climbs by siblings, even a climb by a blind mountaineer. In 2001, two people snowboarded down from the summit.

It is a highly profitable business. A guided tour to the top of Everest costs between £12,500 and £40,000, not including the equipment you will need. There are even plans afoot to open an internet cafe at the Everest base camp.

"I find it all rather sad," says Hillary. "I like to think of Everest as a great mountaineering challenge, and when you've got people just streaming up the mountain - well, many of them are just climbing it to get their name in the paper really."

In one 1993 expedition, 40 people reached the peak in the course of a day, more than did so during the first 20 years after the 1953 ascent. "They might as well have provided a bus," Hillary commented at the time.

"I do believe that many of the climbs on the mountain now lack that sense of success and exhilaration that we gained from going up," he says. "There was a much bigger challenge for us than there has been for later expeditions."

Maybe he is just in a good mood, but as the 50th anniversary of the ascent approaches he is talking with the circumspection of an ambassador. On May 29, he will be surrounded by 800 other Everest climbers, many of whom may be uncomfortable with the gruff views of the pioneer who declared a few years ago: "It's all bullshit on Everest these days."

He never returned to the summit after 1953, setting off instead on fresh adventures to the south pole and the source of the Ganges, before dedicating the rest of his life to charitable work among the Nepalese mountain villages.

He says it is this charity work, and not the Everest ascent, which gives him most pride, although he thinks pride an "awful thing". It would be easy to dismiss the comment as a platitude, were it not for the fresh animation with which he talks about Nepal's mountain people.

Perhaps it is the wisdom of age, but there is no trace of self-aggrandisement or razzmatazz in Hillary. He speaks about his achievements with a phlegmatic honesty. It seems almost unnatural: when so many great climbs have been attempted out of jingoism or personal egotism, he appears to have become a climber purely for the physical and mental challenge.

The house in which he lives in Auckland is beautiful, but far from plush. His wife June flits around making tea and conversation, answering phone calls and proffering oatmeal biscuits.

Next to Hillary's bear-like presence, she seems positively sprightly. She suggests he can take off his jacket now the photographer has gone, and he does so.

He gets up again, his 6ft 2in frame still towering. June telephones for a taxi, and we walk to the bottom of a short staircase leading up to the front door. He is leaning heavily on the banister as he offers his hand.