Media playback is unsupported on your device Media caption Joachim Ronneberg: "I only realised the importance of the mission after the bomb on Hiroshima"

Seventy years ago, a small team of Norwegians was sent from Britain to carry out one of the most daring and important undercover operations of World War II - the aim was to prevent Adolf Hitler building an atomic bomb.

On Thursday, the leader of that team and its last surviving member returned to London to lay a wreath at a memorial and receive a flag that had flown over Parliament.

In glorious sunshine, with the Houses of Parliament as a backdrop, Joachim Ronneberg, 93, sat quietly as the head of Norway's armed forces paid tribute to one of their country's great heroes.

Buglers played and then a minute's silence was observed as a wreath was placed at the memorial on the Embankment to the Special Operations Executive (SOE), the organisation Winston Churchill had tasked with carrying out undercover operations in Nazi-occupied Europe.

The SOE raid Mr Ronneberg led 70 years ago was described as the most successful act of sabotage in the war and one with the potential to alter its course.

We very often thought that this was a one way trip Joachim Ronnenberg, Leader of Operation "Gunnerside"

In the public mind, the raid was captured in true Hollywood style by the 1965 film The Heroes of Telemark starring Kirk Douglas.

That version was not quite true to real life - but the story of the real heroes of Telemark is still extraordinary.

Secret mission

Mr Ronneberg had fled Norway for Britain when the Nazis had invaded, but was determined to return and fight.

He was summoned to an office of the Special Operations Executive just above Baker Street Tube station in London and asked to undertake a special mission.

He was asked to find six men he wanted to take on the mission as soon as possible.

The target of the raid was in Vermork, in a remote part of Norway, home to the largest hydroelectric plant of its type in the world.

Mr Ronneberg says he was never told exactly why they were being asked to destroy it.

Heavy water Heavy water, or deuterium oxide, contains a hydrogen isotope that has a neutron, making its molecules heavier than those in normal water

In nuclear reactors it acts as a "neutron moderator", meaning it slows down neutrons, making nuclear-fission reactions more likely

It is a more effective moderator than normal water as it absorbs less neutrons, meaning natural uranium, rather than enriched, can be used in reactors

This makes it cheaper and easier to extract plutonium for use in nuclear weapons

British intelligence had realised that the Nazis were protecting the plant because it produced a supply of heavy water, which could be used in the production of an atomic bomb.

Direct bombing was ruled out because of the scale of civilian casualties that would result if the liquid ammonia storage tanks at the plant were hit.

A small team known as the Swallows had landed in October 1942.

Their task had been to guide in two gliders full of airborne troops who would destroy the plant.

But this mission ended in disaster.

One of the gliders crashed into a mountain and another onto high ground.

The survivors were executed, but the Swallows survived and lived in the wild, hunting for food.

After three months, they received a message that six more Norwegians would be sent in an operation codenamed Gunnerside.

Mr Ronneberg was to be the leader of the team.

"We very often thought that this was a one way trip," he told me during an interview at Britain's exclusive Special Forces Club, whose walls are littered with pictures of veterans of secret operations.

'Impossible climb'

It was 16 February, deep winter and pitch black, when Mr Ronneberg jumped from the plane.

Underneath snow suits, they wore British battle-dress in the hope it would offer them some protection if captured.

Image caption Joachim Ronneberg (centre) at a service to commemorate his team

If they were known to be members of the Norwegian resistance, they would have been shot immediately.

They carried cyanide pills as well - just in case.

"We jumped out at midnight and the landscape was covered with snow," Mr Ronneberg says.

The team landed in one of the wildest areas of northern Europe and in the wrong location - miles away from the planned site, and it took five days to connect with their reception party.

Eventually they made their way to the plant by night.

There was a bridge across the gorge that the Germans thought was the only way to cross and so it was carefully guarded.

The gorge was steep and dangerous with a river flowing through it.

The men decided to vote on which route to take and the majority voted for the gorge.

"When you look at the gorge where we climbed down, you feel it is impossible," Mr Ronneberg says.

They crept into the factory along a railway line that a local contact had told them was relatively unguarded.

They used wire-cutters to get into the factory.

The doors to the heavy-water plant were closed and so Mr Ronneberg crawled in through an access tunnel.

"Getting inside I was quite certain that the rest of the party would follow me, but only one chap came," Mr Ronneberg says.

"The other ones hadn't found the entrance for the tunnel.

"Therefore we decided we would have to do it ourselves and started laying out the charges."

'Hope of freedom'

Two more men, who had broken a window, eventually joined them.

The explosion when it came was almost disappointing - not quite the huge bang they had expected.

But the escape was the most remarkable aspect of the mission.

Nuclear weapons Nuclear bombs have only been used twice in history, both by the US on Japan

The first, on 6 August 1945, killed more than 100,000 people in the city of Hiroshima

Three days later, the second killed at least 40,000 people in Nagasaki Was it right for the Americans to drop nuclear bombs on Hiroshima and Nagasaki?

It involved travelling more than 200 miles on skis through southern Norway.

An entire German division was sent to chase them across Telemark, with aircraft searching overhead.

With a wry smile, Mr Ronneberg describes it as "the very best skiing weekend I ever had".

The plant was put of action for months.

Later, when it was possible to attack it from the air, American bombers inflicted further damage.

A ferry carrying supplies of heavy-water away from the plant was also sunk.

When did Mr Ronneberg realize the importance of the mission? "That was in August 1945 when they dropped the bombs on Hiroshima and Nagasaki and then we knew what we had done was of great importance. Not until then," he says.

For Mr Ronneberg - the last member of the team to survive - the 70th anniversary is a chance to remember those he served with.

"We were a gang of friends doing a job together," he says.

It is also an opportunity to emphasise the importance of the ties between Norway and Britain forged during the war.

Present at the ceremonies to mark the anniversary were the head of Norway's armed forces and its defence minister.

Mr Ronneberg received a flag which had flown over the Houses of Parliament.

It was a reminder, he says, of the time during the war when he looked up at it flying over Westminster.

The colours had reminded him of his own flag and the fact Britain offered the best hope of freedom for his homeland.

"We felt very much that we had a big debt to Britain," he tells me.

"They received us, they trained us and they helped us. This operation wasn't Norwegian or British at all. It was an Allied operation."