One of this week's more surreal sights has been the reappearance in England of Mark Thatcher, who will be chief mourner at the forthcoming official funeral of his mother.

The now 59-year-old – who was convicted in South Africa in 2005 over his involvement in a failed coup in Equatorial Guinea – has achieved a certain fresh respectability in the eyes of the world, by now being entitled to call himself "Sir" Mark.

The title is a consequence of the bestowal on his father Denis, in Mrs Thatcher's own resignation honours as prime minister, of an unusual hereditary baronetcy. When his father died in 2003, the title passed to Mark: it was thus the last of several favours his mother did for him, thanks to her position.

He is now reported to spend much of his time in the fashionable Spanish resort of Marbella and was relaxing in Barbados when news came of his mother's death. Sir Mark, titular head of the Thatcher family, was speaking earlier this week of the "very sad moment" of his mother's death, in what must have been his first formal interview before the UK's TV cameras since his rather embittered exile from the UK almost 30 years before.

His departure in 1984 followed the first of what were to be many scrapes. A career as a would-be rally driver ended when he got lost in the Sahara desert, and had to be rescued at considerable expense. Then, following a political scandal, it was decided by the then prime minister's PR advisers that Mark would do well to leave the country.

He had been exposed by the Observer for trading on his mother's name in the Middle East, over a contract to build a £300m university in Oman. A company controlled by construction magnate Victor Matthews employed Mark Thatcher as a secret consultant, who would get a large commission if the firm landed the contract. He travelled to the British client state of Oman to join his mother, who was on an official mission there. She strenuously lobbied the sultan to award a building contract to her son's firm, Cementation Ltd.

Her behaviour caused such concern to diplomats that it leaked back in London. In the ensuing political uproar, Mrs Thatcher was unable to deny that she had been well aware of the way her son stood to gain from her conflict of interest. She beat down opposition, however, by claiming she was "batting for Britain".

Behind the scenes, Mark was also allegedly set to benefit from the biggest arms deal in British history, also personally promoted by Mrs Thatcher, in which large secret payments were made by arms firm BAE to cement the so-called al-Yamamah warplane sale to Saudi Arabia. The arms sales to the Saudi ruling family gave BAE a staggering £43bn in revenue over the following two decades.

Syrian-born middleman Wafic Said handled multimillion pound commissions on the deal. Mark has previously denied that as much as £12m of those secret commissions came his way and formed the foundation of his suddenly-acquired fortune. But it has not been disputed that a £1m Mayfair house in Eaton Terrace was purchased on his behalf in 1987 by a Said offshore company. According to his biographers, Mark Hollingsworth and Paul Halloran, Said also gave Mark a £14,000 Rolex watch.

Even Mark's twin sister Carol, who had always sought to earn her own living as a journalist, subsequently described Mark's behavior as "appalling". She told the Lady magazine in 1996: "In some ways he has behaved appallingly. I tried to behave in a vaguely respectable fashion because Mark got such awful publicity."

She added: "I didn't want everyone saying: 'Mrs Thatcher must be awful; both her children are off the rails'."

After the Oman scandal, Mark spent much of his time abroad, pursuing increasingly opaque business deals and establishing offshore tax residences.

He moved first to Dallas, where he married the daughter of a car dealer, and then, following disputes about his tax affairs, in 1996 to south Africa, to the wealthy white enclave of Constantia, on the Cape.

It was there that his behaviour landed him in serious trouble. In 2004 he fell in with mercenary soldier Simon Mann. The former SAS officer was helping organise a coup plot against the tyrannical President Obiang of Equatorial Guinea, and Mark was anxious to join in. Mann describes Mark as both an investor and a "brother-in-arms".

But when Mann finally emerged from five years in African prison cells after the coup's failure, he had nothing but harsh words for his former Old Harrovian colleague. He told an interviewer: "One of the biggest mistakes was approaching Mark Thatcher to put in money. He wanted to be one of the boys, gain power in an oil rich country, impress his mother. He turned out to be a very weak link indeed."

Mark was convicted in South Africa in January 2005 for his involvement in the affair, given a four-year suspended sentence and fined £266,000. His marriage had also ended, with equally harsh words from his then wife Diane about his behaviour and failure to agree a suitable financial settlement with her: "I think he was incredibly selfish, putting his needs for self-fulfilment, greed and lust for power before his family."

She added: "I feel sorry for him. It's as though he was given one of the best seats at the banquet of life and he's blown it."

To add to his woes, he was barred from entry to the US, whence his ex-wife and children had returned, on the grounds of his criminal conviction. His search for an offshore home began to look increasingly forlorn, as Switzerland and Monaco did not give him residence permits. He ended up in Gibraltar, the run-down British colony which offers tax breaks to wealthy people, provided they rent large properties there and pay £20,000 a year.

On the credit side, however, Mark appeared to find new happiness in 2008, when he married Sarah-Jane Clemence, sister-in-law of the Daily Mail's proprietor, Lord Rothermere, in a discreet ceremony in Gibraltar.

• This article was amended on 12 April 2013. The original said Mark Thatcher had married "Sarah Clemence, a wealthy widow". Sarah-Jane Clemence was divorced when they married, not widowed.