Thousands of people walk past this inconspicuous building every day, not knowing the dark secrets, messages and decisions formed within its red brick walls.

The is covered in a blanket of secrets, unsurprisingly so since it's a world where in some of the most dangerous places on Earth. But while these deadly battles may be fought mostly in far-flung and unstable countries, it could be said that the heart of the industry - and indeed, its birth - was right in the heart of London, in one of its wealthiest areas and just a stone's throw from Buckingham Palace.

Mayfair's place in the world of mercenaries and private armies was revealed by David Tompkins, a British mercenary who has led a life so extraordinary it's difficult to believe. In Angola, he was the "explosives man" for a notoriously bloodthirsty mercenary leader during that country's civil war. He was later hired to arrange the assassination of the president of Togo and even to kill Colombian drug lord Pablo Escobar by buying a plane to bomb the private prison Escobar had built himself.

Tompkins now lives a relatively quiet life in Basingstoke, but he told the BBC documentary Storyville: Dogs of War of Mayfair's role in the birth and development of the world of private security, private armies and arms dealing, an industry that began to boom as decolonisation spread through Africa in the 1950s.

During the documentary, the 84-year-old takes a trip back to his old Mayfair stomping ground and sheds an unnerving take on what the area hid in the 1970s and 80s.

"Well we're here in Mayfair, this was the centre of the arms and private military companies in the 70s and 80s," he tells the BBC. "The top of Curzon Street was MI5.

"Deanery Street, there's a Georgian house there that you would never believe in the basement was the showroom of the Directorate of the Supply and Procurement of the Yugoslav arms industry.

"Go back down to Picadally, you've got Heckler & Koch in another unmarked building. The US Defence Intelligence Agency worked out of Roebuck House, Victoria. Adnan Khashoggi, the Saudi billionaire arms dealer, had offices here. Everybody who wanted to get in the game in a serious manner and who could afford it lived or worked here.

"The minute they see that address, they know that you're probably rich, successful and they want to do business with you, without ever having been here."

And there is one building, in particular, just a few streets from the very seat of the UK monarchy, that Tompkins highlights. From the outside, 22 South Audley Street looks like anyother period red brick buidling in London, but the building was the workplace of Sir David Stirling, the founder of the Special Air Service (SAS).

Tompkins said: "He was one of the forerunners of what's today commonly known as private military companies and he used to run his private military operations from here. Millions of people walk by and never realise what that address represents."

Stirling's SAS operated behind enemy lines in North Africa in World War Two but he was captured in 1943 and spent the rest of the war in captivity. After leaving the regular British army in 1947, the Scotsman went on to form various private military operations that were alleged to have deployed all over the world. His firms were even linked with a failed attempt to overthrow the Libyan leader Muammar Gaddafi in the early 1970s. One of them still exists today, calling itself "the world's first private military company".

After his death in 1990, Stirling's former businesses are no longer registered at the offices, with the building now being taken up by a financial services company, though his name is still linked to it in records available online.