EVEN Baroness Thatcher’s closest friends and admirers have long despaired at her unwillingness to concede that her devoted son and heir can be trouble.

To detractors, he is his mother’s Achilles’ heel. To the former Prime Minister he is the loyal son whose motives are deliberately misunderstood.

Quite simply, Lady Thatcher dotes on him. She is much closer to Sir Mark than to his twin sister Carol, a journalist, who has rarely caused embarrassing headlines for the first woman leader of the Conservative Party in the way that he has.

The relationship, as in all families, has not always been easy. Aides at Downing Street recall that the Prime Minister would be charming five days in a row, then scratchy and nervous on the sixth. They would discover that Mark was in the flat upstairs and had been “winding her up”.

While Carol wrote a book about her father and interviewed both parents for a moving television documentary, she can hardly be accused of cashing in on her mother’s fame. She divides her time between her home in Switzerland with her boyfriend, a skiing instructor, and a London flat. By contrast, Sir Mark has amassed a fortune estimated at £60 million through various business deals.

Sir Mark, 51, is portrayed in the media as spoilt, snobbish, vain and difficult, in contrast to his “hard-working” twin. Even privately the twins do not get on. He became Sir Mark on the death of his father, an hereditary baronet. At Harrow he passed only three O Levels but excelled at sports, becoming a schoolboy rackets champion. He moved on to accountancy and part-time motor-racing. There were celebratory pictures of champagne showers after wins at Brands Hatch.

He deliberately chose not to follow a political career: “I couldn’t. Not with Mummy being leader of the party. There would always be people who’d say, ‘He’s riding on his mother’s coat-tails’.”

But if the then 21-year-old thought that he could avoid such accusations by entering the business world, he could not have been more wrong. He regularly caused severe embarrassment to his mother when she was Prime Minister. His business interests have even been raised in the Commons, most memorably in 1984 when it emerged that he had been employed as a consultant by the British firm Cementation, which won a contract to build a new university in Oman.

He forged a lucrative career as a “fixer” who brought businessmen together, receiving a percentage cut of any deal. Some of his business centred on the Middle East. He is a friend of Wafic Said, a Syrian-born billionaire arms broker, who helped to secure the £20 billion al-Yamamah contract with Saudi Arabia. It was the biggest arms contract in British history, signed by Lady Thatcher. Sir Mark was said to have played a key role, and profited, in the deal.

In 1984 he moved to the United States after a succession of blunders which included promoting Japanese clothes when his mother was “batting for Britain”.

But it was Sir Mark who first exposed the crack in the Iron Lady’s facade. When he was lost in the Sahara desert in 1982 on the Paris-Dakar rally she shed her first public tears. The only other time was when she left Downing Street as Prime Minister in November 1990. Sir Mark was by her side.

When it had become clear that his mother was in deep trouble after failing to defeat Michael Heseltine on the first ballot in the Tory leadership contest Sir Mark caught the first flight back from the US.

The desert incident turned into a national joke. Five years later the Daily Mirror printed a map in case Sir Mark got lost on the way to his wedding at the Savoy chapel. He married Diane Burgdorf, a Texan heiress, in 1987 but is fanatical about his privacy and never gives interviews.

In America he worked for Lotus cars as an “ambassador” and later invested in a company manufacturing mobile phones which reputedly earned him £10 million. He was surrounded by a tight circle of Republican friends.

He moved to South Africa with his wife and two children in November 1995 after some of his business dealings in the US had resulted in multimillion-dollar lawsuits.

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