It was the most unexpected telephone call that Gordon Stevens had ever received, summoning him to a Special Air Service office in London. The SAS is notoriously tight-lipped but it appeared that the regiment wanted the television documentary maker to spill some of its secrets.

It was 1985. Stevens had caught the eye of the regiment’s high-ups with his film Secret Hunters which revealed how, between 1945 and 1948, a secret SAS unit hunted down war criminals even though the regiment had been disbanded.

To his astonishment, Stevens was invited by Colonel Sir David Stirling, founder of the SAS, to a discreet hotel in the New Forest. There he met and filmed Stirling and the other surviving founders of the regiment, who put on record for the first time how the extraordinary unit was formed in the north African desert campaign during the second world war.

The idea of a film was later dropped, but Stevens saved the transcripts and has now woven them into a book, The Originals.

Stirling, then 70 and in poor health (although he lived for another five years), was not the gung-ho figure that Stevens had imagined.

“He struck me as a very powerful character,” he says. “But talking to the others he came across as a strange mixture — shy and with a quality of vulnerability, yet totally ruthless.”

Stirling’s “originals” were a group of rule breakers, misfits and mavericks. They included a veteran of the Spanish civil war and an Irish rugby international who had been imprisoned for beating up his commanding officer.

Stirling’s big idea came in 1941 as he lay on a hospital bed in Cairo. He was 24, a second lieutenant who had escaped being court-martialled for cowardice after using his hangovers to fake illness. For once he was officially paralytic, having damaged his spine during his first parachute jump.

He was finding life as a commando frustrating: operations were invariably postponed or cancelled. “I’d gone out with the usual enthusiasm for having a go at the enemy,” he said, “but I wasn’t going to have a go at them if we were going to have a dreary life in the desert.”

Stirling’s concept of the SAS was born out of the commandos’ calamitous experience. Instead of a 600-strong commando unit attacking two targets, he reasoned that 15 units comprising four men could attack 15 targets. By exploiting surprise and guile it would be more effective. His revolutionary idea was that every soldier would be independent.

To sell his big idea Stirling knew that he would have to avoid military bureaucracy and go straight to the top.

He left his hospital bed and limped to Middle East HQ, where he used his crutches “as a kind of ladder” and dodged his way to General Ritchie, deputy commander Middle East, who read his paper and ordered him to assemble his new force.

Stirling had the pick of the disgruntled commando forces languishing in the area. Their felonious aptitudes were put to the test by his first order — to “steal” some quarters in which to train the detachment. The men obliged by absconding with a camp temporarily vacated by New Zealanders. “We stole tents, bars and three marquees. We stole the lot,” recalled Bob Bennett, one of the “originals”.

Appropriating the name “Special Air Service” was Stirling’s private joke. It was already the title of a bogus deception unit of tiny model parachutists with 3ft parachutes. Choosing a motto was more difficult. “We Descend To Defend” was briefly contemplated before Stirling decided on “Who Dares Wins”.