From arms deals in Africa to oil speculation in China - the mercenary world of Mark Thatcher

Mark Thatcher, say those who know him, has always liked two things: the limelight and wealth. But in the recent past the former prime minister's son has been remarkably elusive, hiding his money-making activities from the public eye, keeping away from the minor celebrity events which were once his regular haunts.

Perhaps it is not surprising. He is on the run from Equatorial Guinea and the safest place for him at the moment is Spain's Costa del Sol – the infamous bolthole of Englishmen who have issues with the law.

But The Independent has established that his new-found reticence has not stopped Sir Mark from staying in the oil business, a venture, he declares, he finds hugely lucrative. Like most enterprises he has taken part in the exact details of what he does are rather hazy: "Oil futures", he would say airily when someone asks. " I make judgement calls on prices between when a tanker leaves a port and gets to its destination." The business does, however, take him to Russia, China and Japan; give him bank accounts in Cyprus and London and an extremely comfortable lifestyle in Spain and Britain. While in the UK he often stays at his mother's London house in Chester Square, Belgravia, and has been spotted huddled in "deal-making" meetings.

But then nothing is quite what is seems in Mark Thatcher's shadowy world. The seemingly steady flow of funds should easily pay for Casa Flores, a £1.5m villa he and his new wife rent for £5,500 a month near San Pedro de Alcantara in southern Spain. But that has been unpaid for the past three months, leading his landlord to start legal proceedings. "The man is a crook, not just a crook, but utterly arrogant with it," says Stephen Humbertstone, its owner and a fellow Old Harrovian. "I am afraid everything one hears and reads about him is true, one simply cannot trust him. One shouldn't forget he was found guilty of a crime over that African business."

Sir Mark, 54, was convicted in Cape Town in January 2005, fined £266,000 and given a four-year suspended sentence for his involvement in the coup. He had paid out £280,000 to Simon Mann to be used, it is claimed, for the purchase of a helicopter to transport Severo Moto Nsa, Equatorial Guinea's opposition leader living in exile in Spain, who would then hand over lucrative oil contracts to those who had put him in power in place of the ruler, Teodoro Obiang Nguema. Sir Mark admitted in court that he had paid the money, but said he was under the impression it was going to be invested in an air ambulance service to help the impoverished of Africa. The court fine, according to friends of the family, was paid by his mother.

While Sir Mark hurriedly left South Africa, Mann, incarcerated in Zimbabwe before being extradited to Equatorial Guinea, wrote plaintive letters to his wife, friends and lawyers asking "investors" in the plot including "Scratcher", his nickname for Mark Thatcher, to donate money for his defence. In Malabo, Equatorial Guinea's capital, giving evidence in his own defence at the trial he insisted that Margaret Thatcher's son certainly was "not just an investor. He came on board completely and became part of the management team," alongside Eli Calil, a London-based businessman of Lebanese extraction who was "the overall boss" of the mission. Mr Calil has always denied any involvement.

During the court case, President Obiang said that Sir Mark was "known as a dirty player who lives his life getting himself involved in all sorts of dubious deals... He quickly jumped into this boat and became part and parcel of this plot," and he would be "definitely" brought to justice.

The chances of Sir Mark ending up in leg irons at Malabo's notorious Black Beach prison are highly unlikely as there are no extradition treaties between Equatorial Guinea and most European Union countries. He may be arrested in a country more emollient to Equatorial Guinea's demands during his travels, but that appears to be a chance he is prepared to take. There have also been rumours that President Obiang is prepared to carry out unofficial "extraordinary rendition" and take out a contract to have Sir Mark kidnapped and delivered to Malabo.

His marriage to the daughter of an American millionaire, Diane Burgdorf, had ended in a messy divorce in the aftermath of the coup scandal. There had been instances of infidelity on his part, she said. The involvement in the Equatorial Guinea plot was one of the final straws for the marriage. She said: " He was incredibly selfish, putting his own needs for self-fulfilment, greed and lust for power before his family."

At the end of March this year, Sir Mark married Sarah-Jane Russell, the former wife of Lord Francis Russell, an estate agent and chartered surveyor, in Gibraltar with Lady Thatcher in attendance. The couple are alleged to have had an affair while Sir Mark was still married; the liaison was discovered when Ms Burgdorf hired private detectives.

Ms Russell, 42, herself came from a wealthy background. She is the sister of Viscountess Rothermere and their father, the property developer Terry Clemence was a multimillionaire.

Despite all his attempts to portray himself as accidentally getting caught up in the coup plot, Sir Mark could not escape the suspicion which followed him around. Sir Bernard Ingham, Lady Thatcher's press secretary and a fierce loyalist when it comes to the former prime minister, says it was "very difficult to believe" that Sir Mark did not know about the coup plot. "He is not the brightest boy, but by God he knows how to make money. The plain fact is, he's a barrow boy."

Sir Mark is said to have made £20m from the Al Yamamah arms deal when Britain, with Margaret Thatcher as prime minister, sold the Saudis £40bn worth of weapons. According to The Sunday Times Rich List of 2006, his fortune is estimated at £64m, and, he has told friends that he is making a nice living out of his oil deals.

Mr Humbertstone, the landlord in Spain, is unimpressed by all the talk of wealth. "If he is so good at making money, why doesn't he pay his rent? We met at the Carlton Tower [hotel, in London] to discuss the matter, I have repeatedly telephoned him. He says he is in China, sounding very self-important, but the money does not come through. This guy certainly likes to present the image that he is well off, he and his wife have a driver, a cleaner and a gardener. One wonders how many other people he is not paying."

The villa, with four bedrooms and an indoor swimming pool, is in a lush forested area in the El Madronal complex. Security is tight, with CCTV cameras, electronic gates which can only be approached through designated paths, and a state-of-the art alarm system. Residents, say the caretakers, value their privacy although no one else is known to be a possible target of an Equatorial Guinea snatch squad.

Sir Mark was introduced to Mr Humbertstone as a prospective client by an estate agency in southern Spain. "I knew he was the same school as me, in fact the same house, although he was a bit older. Of course I had heard about things he had been up to, but this was going to be a straightforward business deal.

"I agreed to an 11-month contract, but made it clear that I did not want to rent the place out for any more than two years. But then Thatcher said he wanted to extend it to five years and there was nothing I could do about it. There is fine print in local law, which is a matter of dispute, that because he had stayed over 11 months he has rights as a sitting tenant. If he does not pay the back rent he can be evicted."

Mr Humbertstone claimed that there has also been damage to furnishings in the villa. "There was gilt mirror ... I heard had been taken away somewhere in the back of the gardener's van. There are plenty of other examples of this kind of behaviour, this kind of thoughtlessness."

After their divorce, Ms Burgdorf took the children back to her homeland, the United States. Sir Mark cannot visit them there; his conviction in South Africa precludes him from getting a visa. Their daughter, Amanda, then 12, wrote to President George Bush asking for clemency over entry into the country. "You know how you feel about your daughters? I want my Daddy back in America," but there was no response. Michael, his son, who is now 19 – whose birth was announced by Margaret Thatcher as "we have become a grandmother" – has won praise as a schoolboy football star in Texas.

At a bar in St James's Street, a stone's throw from the Carlton Club, the bastion of high Toryism where he would once go as his mother's son, an acquaintance of Sir Mark, who has known him for 18 years, mused: "Is he happy now? Very difficult to say. He says things like he has connections in South African intelligence. Who knows if that's true. The thing is he has been very lucky, managing to get out of sticky situations. I suppose that's one of the reasons behind his arrogance.

"He certainly says he makes money out of oil, although what he does is a bit hazy. But then you also hear that his mother paid his fine at Cape Town and actually sold a couple of paintings to do so. Why couldn't he pay his own fine?"

Mr Humbertstone's views on his tenant are trenchant. "Judging by my own experience, my advice to anyone plotting any future coups is don't depend on Mark Thatcher, he is not the most reliable of people."