Sandy Allan and his climbing partner Rick Allen were at their last camp below the summit of Nanga Parbat, the world's ninth highest mountain, when they gathered together the food they had left.

After 14 days on the vast unclimbed Mazeno Ridge – the longest in the Himalayas – it amounted to a packet of McVitie's Digestives biscuits.

A lighter they had taken with them had stopped working, its flint worn through. The only spare, they realised, was in the pocket of another member of the team who, exhausted and demoralised, had already decided to descend.

The consequence was that they could not light their stove to boil snow for water, essential on the world's highest mountains where the consequence of dehydration is the risk of frostbite and pulmonary and cerebral oedema. They hoped, however, that they could reach the summit and descend the imposing Diamir Face in a day and a half; they were counting on fixed ropes left by other expeditions. In reality, it took them three full days to get down from the summit without any food or water, a lengthy struggle through deep snow at the limits of survival.

Photographs of the two men at base camp indicate what they endured in conquering the last unclimbed ridge on the world's 14 peaks of 8,000 metres – a 13km long undertaking that had rebuffed all previous attempts. Rick Allen, seated drinking tea, appears skeletal. Wiry before he started, his body was starved by the effort.

In the midst of the Olympics, the achievement of the two men in climbing the ridge dubbed the "last great problem of the Himalayas" passed almost unnoticed. In the months that followed, it has become recognised as the most impressive British ascent on an 8,000m peak in a generation, the first major new route since the first ascent of the Kangshung Face of Everest by an Anglo-US team, including Stephen Venables, in 1988.

Now that their story can be told in full for the first time, it is even more remarkable, not least because of their ages: Allan was 57 and Rick Allen 59 when they made their climb.

The ascent of Nanga Parbat along the Mazeno Ridge. Photograph: Doug Scott

Allan, a mountain guide from Newtonmore in the Cairngorms, and a rope access specialist in the offshore oil industry, gave a lecture on his ascent in London last week.

He has guided on Everest and climbed the North Face of the Eiger, but Mazeno – which he first attempted on an expedition led by Doug Scott in 1992 – had stayed in the back of his mind.

First explored by British mountaineer Albert Mummery, who died on the mountain in 1895, Nanga Parbat became the focus of German and Austrian climbers between the first and second world wars and after 1945; it was finally conquered by Hermann Buhl in 1953.

Involving both technical climbing and massive endurance, the Mazeno route requires ascents on seven subsidiary summits above 6,000m before tackling the final summit.

The route had almost been climbed in 2004. Then the US team of Doug Chabot and Steve Swenson climbed all the subsidiary peaks to reach the shoulder below the final 2km of slopes to the summit. But with bad weather and Swenson ill, they had retreated.

"It had always inspired me," said Allan last week. "I had talked about going back again with Voytek Kurtyka [the Polish climber], but for personal reasons it did not happen. I'd been on it before, climbing three of the subsidiary summits before we hit a brick wall. I knew what was needed to climb it."

The team he put together with Allen included three Sherpas and Cathy O'Dowd. The other four team members would become exhausted and "discouraged" by the scale of the ridge, electing to descend before the final ascent.

"When we got to the whale back ridge at the beginning people were saying, 'Wow, this ridge is enormous. Jeepers – it's a long way'. You would cross one of the summits and there was just more and more of it going on."

Even with the most optimistic assumptions – planning for eight days' food and fuel they thought they could stretch to 10 – the climb would take 18 days with seven different camp and bivouac sites, some of them no more than snow holes. "We didn't think it would take 18 days. I'd done the Diamir Face [the route they would descend down] two years ago. I thought … we could get over the summit and down in a day and a half. Instead we spent three nights on the descent."

During that descent, without food or water, and with both men suffering from frostbitten feet, and slowed by deep snow, they were in danger .

"Cathy had left us the satellite phone when she descended. We had enough power for three calls. I was getting concerned the day after the summit. Rick was very tired. I called our agent to check what availability there might be for a helicopter if we got in trouble. I was worried and said we had the potential to be in trouble. Rick heard the call and rallied. He's a tough and determined individual. But he was very tired, sitting down to rest every few minutes, falling asleep sometimes. On the last day, which involved steep icy ground, what should have taken between three and four hours took us all day, getting into camp at 11 o'clock at night."

Among those who praised the ascent was Doug Scott who described it as an "incredible achievement".

The challenges of the ridge have been detailed by mountain guide Mark Synnott. "Although the ridge itself is objectively safe," he wrote in a US climbing magazine, "it's important to remember that the route's inordinate length makes it a very committing proposition. Both sides are guarded by huge Himalayan faces that you wouldn't want to abseil in your worst nightmare, so if you were less than halfway out, your best bet would be to backtrack – time-consuming and difficult in bad weather. Naturally, there's a point of no return."

The question is why was there such a long gap between the mapping of British routes in the Himalayas. While some new routes have been opened up by the likes of Alan Hinkes, the first Briton to climb all the 8,000ers, the gap has been noticeable.

Allan ascribes this historic change in emphasis to a series of high-profile disasters for British mountaineering, beginning with the disappearance of Peter Boardman and Joe Tasker on Everest in 1982. "I think with the loss of Boardman and Tasker and Al Rouse on K2, there was a realisation that pushing big routes on big mountains could be very dangerous."

Asked about the future he answers: "I'm 57. I'm a father. The rat's been fed."

Rick Allen will be giving a lecture on the ascent at the Rheged Centre, Penrith, on 2 December (enquires@rheged.com)