A secret World War II diary of the British special forces unit, the SAS, has been kept hidden since it was created in 1946. Now it's being published for the first time to mark the 70th anniversary of the regiment. The BBC has exclusive access to the remarkable piece of history.

It was 1946; World War II was over and so was the Special Air Service, better known as the SAS.

Set up in 1941 by David Stirling, a lieutenant in the Scots Guards at the time, it had changed the way wars were fought, dispensing with standard military tactics and making up its own. But in the new post-war world those in charge no longer saw a need for the regiment. It had been disbanded and there were no plans to revive it.

But for one former SAS soldier it wasn't over. Determined that the regiment's story wouldn't fade away and become a footnote in history, he made it his job to find and preserve whatever documents and photographs he could before they were lost forever. It was his final SAS mission.

The PoW and the Nazi general Major J Tonkin was captured in France on 3 October 1943. He filed this report after escaping: At about 1100 hours a corporal brought me water to wash in and said that General Heidrich wanted to see me and that it was his custom to entertain all British paratroop officers whom he captured. Heidrich was a man of medium size, rather chubby, with light hair and pale eyes of indeterminate colour. He was inclined to be bald, and although pleasant enough, gave one the impression that he could be ruthless. Topics of conversation were obviously going to be tricky, but he started off with a formal invitation to lunch, and would I like chicken or pork? He added that it was immaterial which one I chose, as it was "borrowed" from the Italians, and of course I would think it looting. Being hungry I hastened to assure him that I called it "living off the land" when I partook of the deed. So chicken it was! The last subject he talked about was what a beautiful stroke the Termoli landing had been. It had inconvenienced them a great deal and was perfectly timed. Then the German corps commander came in and I was taken away. The next day I escaped while being moved.

As it turned out the elite force's expertise was still needed and it was resurrected just a year later in 1947. And by then the soldier's personal mission had resulted in something unique - a diary of the SAS in WWII.

Unorthodox from the start, the SAS was conceived as a commando force to operate behind enemy lines in North Africa, where the British were fighting Field Marshall Rommel's highly-skilled Afrika Korps. Their orders were to attack enemy airfields and harass the Germans in any way possible. Over months they repeatedly went into the desert and destroyed German planes, sometimes with bare hands when their bombs ran out.

After the end of the North African campaign, the SAS then served in Italy. It was at the forefront of the action with the Normandy landings in June 1944, again going behind enemy lines in jeeps assisting the French Resistance and providing crucial intelligence for allied forces.

The SAS continued to be at the forefront of operations through Belgium, Holland and Germany until the end of the war in Europe.

Documents in the diary include the top secret order authorising the first SAS operation and rare photographs of the team which carried it out, naming those who died. It also had highly-confidential briefing instructions to kill Rommel in France. He was injured and sent back to Germany before a team of four SAS men reached him. There was confidential correspondence from Prime Minister Winston Churchill on the future of the regiment and the order assigning it regimental status.

It was a huge tome. The soldier had bound everything in a single, leather-clad book which totalled 500 pages. It measured 17in (43cm) by 12in and weighed over 25lb (11.3kg).

But having created something unique, he then stored it away at his home for more than half a century and told no-one about its existence. Coming from a regiment where discretion was part of its ethos, and belonging to a generation of men who were reticent to talk about their war experiences, it would have been the natural thing to do.

Media playback is unsupported on your device Media caption Military historian Gordon Stevens takes Robert Hall through the diary

It was only in the late 1990s, shortly before his death, that he took it to the SAS Regimental Association and handed it over. It was then put in the regiment's highly-confidential archives for years, where only a handful of people knew about it.

Its existence was only revealed outside the SAS when documentary maker and writer Gordon Stevens stumbled across it. He had worked closely with the association on several projects and asked to look at photos from its archives. The diary was brought out and it took him just seconds to realise how important it was.

I would have done the same thing as that man and put the diary away in a cupboard, I still would today Mike Sadler, SAS veteran

"As soon as I saw it I knew it was an incredible document," he says. "The records in it don't exist anywhere else. From its contents to how it was pieced together, it was astonishing."

After two years of negotiation it's now being reproduced and published for the first time to mark the 70th anniversary of the SAS. Limited numbers will go on sale at £975 each, with most of the proceeds going to the association.

Months of work have been put in to include material not available to the soldier in 1946, and now held in the association's archives. The pages have been ordered chronologically and reports, maps and photographs have been added to complete the picture and tell the full story of the wartime SAS.

"The diary is a unique document and going through it is a very humbling experience," says the executive vice-president of the SAS Regimental Association, Col John Crosland, 64. He worked on the project and was one of the few people who knew about the diary's existence at the association.

"It shows how extraordinary these men were. Their deeds were astonishing but they are so matter of fact in their reports. What they did with the little kit they had was phenomenal. Their radios probably weren't very exact and medical recovery would have been non-existent."

Image caption Paddy Mayne led 1 SAS after Stirling was captured

Much about the diary still remains a mystery. The regiment is not naming the soldier who put it together and little is known about how he got hold of so much important information. Some have speculated that SAS founder Stirling may have encouraged his men to contribute, but those alive today think it is unlikely.

"I had no idea someone was putting the diary together," says 91-year-old Mike Sadler, who was 21 when he became a member of 1 SAS and Stirling's navigator.

"When the regiment was disbanded after World War II we all went our different ways. Anyway, we never spoke about what we did. We just didn't think that way and still don't.

"I would have done the same thing as that man and put the diary away in a cupboard, I still would today. The thought of publishing the diary would not have crossed our minds."

Its publication is very significant, says military historian Antony Beevor, the author of many books including D-Day: The Battle for Normandy. He says up until now there has been very little material about the birth of the SAS.

Media playback is unsupported on your device Media caption SAS founder Colonel David Stirling, speaking in 1985 about the men who served under him in the early days

"That generation of men just didn't talk about their experiences so there is very little information around. They had a huge respect for things like the Official Secrets Act and the SAS were even more security conscious than most.

"The regiment has always fascinated people. It is the most extreme form of military life imaginable."

So why publish it now? Despite the interest in the SAS, talking publicly still doesn't come naturally to those involved with it - past and present. The diary is about celebrating the regiment's 70th anniversary, says Col Crosland. But it is also about educating people - including those in the forces.

"Even within the military, people are ignorant of the part played by the fledgling SAS in World War II. Nearly every allied operation was led by the SAS or the SBS (Special Boat Service). These men were sometimes dropping 500 miles behind enemy lines."

The early SAS The SAS began life in 1941, the unorthodox idea of Scots Guards Lieutenant David Stirling

In September 1942 it was officially designated 1st SAS Regiment

Stirling was captured in January 1943 during SAS operations in southern Tunisia

After escaping several times he ended up as a prisoner in Colditz Castle

For his distinguished actions while a prisoner he was made an OBE

In May 1943, his brother Lieutenant Colonel William Stirling raised a 2nd SAS Regiment

Both were disbanded in October 1945

The SAS was reformed in July 1947 Source: SAS Regimental Association

There is also an increasing awareness that time is slipping away, with SAS veterans from WWII getting older. According to the association, 143 are still alive today, including veterans from the SBS and other small units that came under the SAS at the time.

"We thought about publishing the diary for the regiment's 75th anniversary, but knew even fewer veterans would be alive," says Col Crosland.

The association's own archivists have been working closely with older members to extract their stories. But even then the accounts are kept very much as private regimental mementos.

For former SAS members like Sadler, this is the right thing to do.

"Even today I think twice when it comes to speaking about my experiences," he says.

But what the regiment does hope is that the diary may prompt other people with documents or photographs to come forward. These can then be added to its archive.

"This will always be a work in progress," says Col Crosland.

And the soldier who started it all?

"Ultimately, the story of the SAS in World War II is about more than just one man," says Stevens. "I think he would have agreed with the decision not to name him."