WHEN Mark Thatcher asked his mother's press secretary how he could best help her re-election prospects in 1987, Sir Bernard Ingham's reported reply was as brutal as it was short: "Leave the country".

Sir Mark, 51, who inherited his baronetcy on his father's death last year, had long been an embarrassment to the then prime minister, from getting lost in the desert to his controversial business dealings, giving him a reputation which has only been less well publicised rather than diminished since Lady Thatcher's fall from power.

But his arrest in connection with an alleged attempted coup goes well beyond embarrassing. Now, an investigation into claims he helped fund a military adventure in a small west African country looks likely to expose his business affairs to unwanted scrutiny, as well as potentially landing him in jail.

Until his arrest at his home in Cape Town yesterday, he was probably best known for getting lost in the Sahara desert in January 1982. The 28-year-old motoring enthusiast had been taking part in the Paris-Dakar rally when he went off course, near the border between Mali and Algeria.

His mother was said to be "very upset and very distressed" as a full-blown search initially failed to find any sign of his white Peugeot 504. Then, six days after going missing, the car was spotted by a Hercules search plane belonging to the Algerian army, just as he, with his mechanic and co-driver, was running short of food. He was flown back to the UK in the Algerian presidential plane, with his relieved, and somewhat embarrassed, father. Mark himself seemed unaffected by the ordeal, and said all he needed was "a beer and a sandwich, a bath and a shave".

He later admitted he had been under-prepared for the rally, but the experience prompted him to write: "The biggest story of 1982 was the Falklands War. The second biggest also involved my mother and me."

Mark had not excelled academically at Harrow, where he was known as "Thickie Mork", but he did shine at racquet sports and developed a taste for fast cars.

He left school with three O-levels and failed his accountancy exams three times. He drifted through a succession of jobs, even trying his hand at motor racing, but a couple of bad crashes persuaded him he had no future as a professional driver. Even his motor racing company, Mark Thatcher Racing, failed to last the distance after it developed cash problems.

But it was as he moved into business that he began to cause his mother, by then prime minister, embarrassment.

In 1984, the Observer revealed that he had represented a British company, Cementation, which had won a contract to build a university in Oman, shortly after his mother visited the Gulf State and urged them to push work Britain's way.

Mrs Thatcher denied there was a conflict of interest and insisted she had merely been "batting for Britain", but her son severed his links with Cementation and moved out of his Downing Street flat.

Two years later, the Prime Minister again faced questions over her son's business activities, this time over his links with the Sultan of Brunei, and a series of arms deals in the 1980s, including a £20bn contract between British Aerospace and Saudi Arabia, raised eyebrows.

It was perhaps in an attempt to avoid these embarrassing headlines that Mark took Bernard Ingham's advice and indeed left the country, moving to the United States where he dated a string of women before marrying Diane Burgdorf, millionaire heiress to a second-hand car dealership.

The couple married on St Valentine's Day in 1987 and it was the birth of their first child, Michael, in 1989, which prompted Mrs Thatcher's much-mocked exclamation, "We are a grandmother".

Living abroad gave him the lower profile his parents must have hoped for, but did not keep him out of the headlines altogether.

His business interests meant he lived for a time as a tax exile in Switzerland, but the failure of a security alarm business and prosecution for tax evasion in the US prompted him to up sticks once more, this time to Cape Town, in 1995.

Three years later, he came to the attention of the South African authorities when he was investigated over lending money to police officers. It was alleged that when they defaulted on the loans, Mark put debt collectors on their trail and charged them 20 per cent interest.

Despite the controversies, his business dealings are believed to have made him a personal fortune of £60m, attributed to a series of investments and deals in Africa.

He was first linked to the alleged coup attempt in Equatorial Guinea, a country of just 500,000 people which became rich with the discovery of oil several years ago, by its ruler, President Obiang, who himself came to power in a coup in 1979.

It was in March that a plane with 64 alleged mercenaries on board was impounded by the Zimbabwe police as it flew in from South Africa. Another 15 men, said to be the advance party and led by Nick de Toit, were detained in Equatorial Guinea.

De Toit told his trial that he had been recruited by Simon Mann, the founder of mercenary firm Executive Outcomes and a friend of Mark Thatcher's. President Obiang has accused the Spanish government, a former British cabinet minister, who he refused to name, and oil tycoon Eli Calil, of being involved, as well as Mark Thatcher. The former prime minister's son has denied any involvement.

He returned to Britain briefly last year for the funeral of his father Denis, on whose death he became Sir Mark, after John Major had controversially revived the hereditary baronetcy for the former prime ministerial consort.

The honour was said to have been in response to a personal request from Lady Thatcher, whose devotion to her son has always been absolute. Despite his reputed lack of charm, she seems to have always believed he can do no wrong, to the chagrin of his much more grounded twin sister, Carol.

Such is Lady Thatcher's faith in her son that she is reported to have said: "Mark could sell snow to the Eskimos, and sand to the Arabs." It now seems he is in need of this apparent talent of persuasion as never before.