Sir Mark and Carol Thatcher will be forced into the limelight one last time for mother's sake

Wednesday will be the most trying of days for Sir Mark and Carol Thatcher. Not only will they be bidding farewell to their mother at her funeral, but they will do so in most public way imaginable.

The children of Margaret Thatcher, born two minutes apart 59 years ago, have received plenty of attention over the years, some of it welcome, a lot of it not. Dissimilar in many ways, they have learnt to avoid publicity, preferring to live in obscurity overseas.

But that desire for privacy will have to be put aside as brother and sister take their positions behind the gun carriage at the ceremonial funeral of the longest-serving prime minister of the 20th century. Even in death, the Iron Lady exerts a powerful influence on her offspring: now they must play their part in the last act of a titanic political career that so often overshadowed their own lives.

"My mother was blessed with a long life and a very full one," said Mark, as he addressed the cameras outside his mother's home in Belgravia on Wednesday, two days after her death at the Ritz hotel. Public statements do not come naturally to the 2nd Baronet, and his hand-wringing did much of the talking, but his words were eloquent enough.

Margaret Thatcher's life was indeed full, so much so that it admitted little time for family.

"Somehow, even as a new mother of twins, Mum found time during her two-week stay in the hospital after our birth to plan the next stage of her career," observed Carol in her autobiography, A Swim-on Part in the Goldfish Bowl. "To many people, I wasn't really an individual so much as an adjunct to my mother."

Self-sufficiency was the order of the day as Margaret forged ahead in politics. Mark was dispatched to boarding school at eight and Carol at nine. Insecurity manifested itself in distinct ways - outward arrogance masking fear of failure in Mark's case; a kind of hearty female bufferdom in Carol's.

"Family rushed past us, eclipsed by the sheer pace of my mother's career as she worked her way up the greasy political pole," Carol has said. "And my father's as well, when he was running his own family business. We never had long, relaxed, talking Sunday lunches."

The twins' lives quickly diverged. Mark flunked his O-levels at Harrow and repeatedly failed his accountancy examinations. He is said to have considered applying for a commission in the Irish Guards but nothing came of it. Carol was better at exams, gaining a place at University College London to read law before opting for journalism. Their mother's election victory in 1979 ensured their careers would be public property from then on.

Mark's first collision with the press came in 1982 when he managed to get himself lost in the Sahara during the Paris-Dakar rally. There would be plenty more as he played on his connections to amass the fortune he coveted, notably in deals on the Arabian Peninsula and in Africa.

"Mark loves to be thought of as having a barrow-boy mentality," says Simon Mann, the former SAS officer and one-time mercenary. "But he is a trader, wheeler-dealer, sharp as tack. He's a commercial helicopter pilot, fully licensed. If you talk to him about aircraft, shipping, commodities or other lines of business, he really knows what he is talking about. He's no fool."

What drives him? "Money." And adventure. After marrying Diane Burgdorf, daughter of a Texas car dealer, Thatcher grew weary of America and moved with his family to Cape Town. There he befriended Mann and his wife, Amanda.

"We did become quite friendly," says Mann. "I rather liked him. He could be fabulously rude on social occasions, which I thought was very funny. He had this rhinoceros hide. Diane was a lovely woman, very beautiful, very bright, an ernest Christian, and obedient."

In 2004, Thatcher agreed to part-finance a mercenary operation led by Mann and intended to overthrow Teodoro Obiang Nguema Mbasogo, the despotic ruler of oil-rich Equatorial Guinea.

"He loved the whole SAS thing, mercenaries running round Africa," says Mann. "And the possibility of making a lot of money."

As well as putting up money for a helicopter, Mark is said by Mann to have agreed to take part in an operation to extract the mercenary force, should things go wrong. Disaster duly followed. Mann and his team were detained in Zimbabwe and he was subsequently sent to Equatorial Guinea for trial and imprisonment. Thatcher was arrested in South Africa and pleaded guilty to violating that country's anti-mercenary law, receiving a four-year suspended sentence and heavy fine. Mann says he and his men were abandoned to their fate as Thatcher sought to save his skin.

"Mark Thatcher absolutely failed to do anything for the men's families, my family," says Mann. "There were hundreds of things he could have done to help. He did none of them. On the contrary, his statement to the South African authorities really screwed me up, screwed everyone else up."

Mark's marriage, already in trouble, did not survive. In 2005 he and Diane divorced, she having already returned to the US with son Michael and daughter Amanda.

In 2008 Thatcher married Sarah Jane, formerly Lady Francis Russell, with whom he had conducted an affair during his first marriage. The couple married in Gibraltar, prompting speculation that Thatcher was planning to live there as a tax exile, but he has spent recent years living in gated seclusion in southern Spain or at a pounds 3 million villa on Sandy Lane, the most exclusive thoroughfare in Barbados. Both Diane and Sarah Jane will attend the funeral, together with Michael, like his mother a committed Christian, as well as ardent Republican, and Amanda.

The origins of Thatcher's wealth have always been opaque. "There is plenty of money," says Mann. "Full-on staff in the Cape Town house - butlers, chefs, maids, the whole thing. I did notice that all the money was in trusts, very well organised, very well hidden. For example, when he had to put money into the coup plot, he had to get that money out of a trust. He couldn't just whip out a cheque book."

Carol has often expressed dismay at her brother's escapades and the two enjoy a distant relationship, not helped by the perception of Mark as the family favourite.

"In some ways, he has behaved appallingly," said Carol - and that was before the so-called Wonga Coup. "I tried to behave in a vaguely respectable fashion because Mark got such awful publicity."

By and large, she has. In 2005 her down-to-earthness won her I'm a Celebrity… Get Me Out of Here! but she fell from grace after being heard describing a black French tennis player as a "golliwog". Depressed, she retreated into the arms of her on-off boyfriend, Swiss ski instructor Marco Grass. The two share a flat in Klosters.

A friend of Carol's says of her: "She enjoys skiing and swims regularly but she can be lazy. She's like her mother, though, in that she loves a good argument. She will sit there for hours at the bar and argue just for the sake of it."

Linda McDougall, who spent a year with Carol producing a documentary on Denis Thatcher, is a fan. "Carol is terribly honest, very attractive and lovely," she says. "She's also very smart and very lonely. She doesn't trust anyone because she feels people let her down. She doesn't have many friends - not that I saw. She is a much nicer person than the (golliwog incident) suggested."

Unlike Mark, who visited his mother regularly as stroke-induced dementia took its toll, Carol remained distant from her mother.

"I had to make appointments to speak to her," she once said of her mother's time in Downing Street. "I didn't find it difficult because it was absolutely the norm."