Cape Town is popular at Christmas. From mid-December to mid-January, South Africans decamp from Johannesburg, Pretoria and other landlocked cities and head for the beaches, cafes and hotels of the “Mother City”.

Most years when he lived in the luxurious enclave of Constantia, Mark Thatcher threw a house party. His parties were well attended, especially when his mother was present — as became increasingly frequent after her husband died. But his neighbours and guests were hardly grateful. “If it wasn’t for his mother, he’d be an East End barrow boy,” said one.

Another described him as having “an ego the size of a herd of elephants and the attention span of a gnat”. He was said to be rude to waiters and imperious to everyone.

Greg Wales, a British businessman who has long pursued his fortunes in Africa, vividly recalls flying to Cape Town from Johannesburg shortly before Christmas 2003 to attend Thatcher’s party. It was an exhilarating journey in a private plane. The South African pilot, Crause Steyl, took him down into a narrow gorge in the Drakensberg mountains, racing low inside the canyon and twisting the plane sideways to bring them through.

Also enjoying the ride was Simon Mann, a former SAS officer and adventurer. Most who have known him rather like this upper-class Briton; women found his old Etonian manners endearing. A South African who met him socially thought him “quite charming, not too dominating. Physically he is not a hulk, he is not a marine, but smallish, slim-boned”.

Wales, Steyl and Mann were old friends with a mission. They were going to spend Christmas and the New Year putting the final touches to plans for a coup in Equatorial Guinea — one of the nastiest oil-rich nations in Africa — that they hoped would make them hugely wealthy for the rest of their lives.

Several other characters, either engaged in the “Wonga Coup” or very much aware of it, were also in the Mother City that Christmas.

Among Thatcher’s house guests was a jovial, sandy-haired individual called Nigel Morgan, a former member of the Irish Guards whom Thatcher had earlier helped through a difficult patch in South Africa by providing him with a home for a spell.

Morgan’s story is one that Graham Greene might have relished. He trained briefly as a Jesuit priest after working for a think tank that advised Margaret Thatcher in the 1980s.

Known to Mark Thatcher, Mann and other friends as Nosher or Captain Pig, Morgan has a startlingly red face, having spent years under the African sun while swallowing tumblers of pink gin and whisky. His love of hearty English food and cigars is matched only by the pleasure he takes in spinning yarns and arguing about politics. He would later be accused of betraying his friends, though he denied it.

Morgan is not simply a bon vivant. He trades intelligence. When he first heard whispers of a plan for mercenary action in west Africa he suspected that his friend Mann was involved. He resolved to get close to the action. As a freelance intelligence man he wanted a chance to make money. This could be a moment to do business in Equatorial Guinea with Mann running things. On the other hand, he had close ties with the South African authorities and would be expected to pass on what he knew to them.

Morgan managed to get James Kershaw, a young South African friend who was a capable administrator skilled in electronic communication, a job as Mann’s personal assistant. Although Kershaw says he had no idea what was going on, he effectively became a mole at the centre of Mann’s operations.

Mann probably knew, but did not mind, that old Nosher got some information from Kershaw. He may also have suspected that Morgan relayed some of it to the South African authorities. Perhaps he hoped that, if the government severely disapproved, it would pass a message of discouragement back to him.