Simon Mann has been urged by Foreign Office officials to remain silent about the coup attempt that left him languishing in an African prison, and settle for a "quiet life" with his wife and family in the UK, The Independent on Sunday has learnt.

The veteran mercenary returned to Britain last week after he was pardoned by oil-rich Equatorial Guinea's President Teodoro Obiang Nguema – the man he had planned to overthrow five years ago. Mann, with the gratitude of a man sprung 34 years before his sentence was due to run out, apologised for the plot that ended with his incarceration in the notorious Black Beach jail. He swiftly made it clear he wanted revenge on those he believes made him the "fall guy" – notably the Lebanese millionaire, Ely Calil, and Sir Mark Thatcher, son of the former British prime minister.

Mann's friends confirmed yesterday that he wanted "justice" for both men – not only for allegedly leaving him to carry the can for the disastrous coup attempt, but also for failing to look after his wife and children while he was in captivity thousands of miles away.

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Yet they also revealed that Mann has already been subjected to government pressure to keep his mouth shut. "The Foreign Office didn't do anything to help get him out of that place, but they have been very quick to try to get him to play ball now he is back," one close friend said. "Simon has been told it would be in everyone's best interests if he could just draw a line under this whole thing. We know the Foreign Office wants to get on-side with EG [Equatorial Guinea] as quickly as possible but, frankly, it is also in their own interests for people to stop asking questions about this whole affair."

Jack Straw, Britain's foreign secretary at the time of the attempted coup, initially denied that the Government knew about it in advance, but was later forced to admit that he did know. Whether any attempt was made to stop it, or encourage it, is not known. Mann has claimed that the UK, US, and Spanish governments all had prior knowledge.

It is clear that, despite the blissful photographs with his wife, Amanda, in the New Forest, Mann's return home is no neat and happy ending to the sorry saga. For many individuals, organisations and foreign governments, it could initiate an uncomfortable fresh chapter as questions are asked about the circumstances behind the audacious attempt to depose a hardline ruler and take control of his nation's oil supplies.

This weekend, as Mann ponders going public with his story – via a newspaper buy-up or, eventually, a book deal – the first significant questions over the credibility of the "coup plot" are beginning to emerge. Not least among them is whether the operation was ever a real "goer", as one critic described it: how an experienced former SAS man seriously expected to capture an entire state with just 60 men, and why stopping in Zimbabwe en route was deemed a sensible part of the strategy. Their plane, a Boeing 727, was reportedly on the military side of the airfield, and beside it were 50 heavy machine guns, 20 light machine guns, 100 rocket-propelled grenade launchers, 61 assault rifles and tens of thousands of rounds of ammunition. Inconspicuous is not a word that leaps to mind, which, in turn, suggests possible explanations. It was either a bafflingly naive diversion for a team so steeped in the ways of Africa to make. Or the conspirators thought the necessary people in Zimbabwe had been squared.

Reports in the Daily Mail claim that the call then went out for Sir Mark Thatcher to use his influence to get Mann and co liberated. Sir Mark, the paper says, had done oil deals with the Mugabe regime, and often claimed to have considerable pull with the country's leadership. But Sir Mark – famous for translating his three Harrovian O-levels and the helpfulness of his surname into a successful life as a trader in commodities (and with connections that have never entirely been explained) – allegedly declined to help, and Mann languished in prison.

At around this time, word seeped out that Sir Mark was involved with the plotters. So, one fine, cocktail-sipping day, he was arrested in his luxury home, tried for financing a helicopter that would have been used in the coup, and fined. Sir Mark claimed he thought the money was for an air ambulance. After Mann had been shipped to Equatorial Guinea for trial, he further alleged that Sir Mark, far from being the bankroller of a chopper, was in fact "administrator" of the putative coup.

Now Mann is back and, whether civil servants or his former allies like it or not, he is determined to make sure no one will be able to forget the escapade. It is clear his former captors share that view. "One should not underestimate the extent to which the EG government wants this to be the start of a process that gets to the truth about this operation," one of Mann's former colleagues said. "They want an 'in' with the world community, but not at the expense of getting at those who tried to undermine them. Simon was genuinely freed on compassionate grounds, but he co-operated with the EG authorities and told them just who was responsible for this whole operation."

This declaration will fray the nerves of Mann's alleged comrades. Mr Calil, who denies financing the plot and Sir Mark have both welcomed Mann's release in recent days; Mr Calil, dubbed "The Cardinal" by Mann, has even expressed the hope that "we can talk". He has the most to lose – in terms of his reputation, at the very least – if he were linked to the plot. But his friends maintain he has already been assured by British authorities that he would face no action over the affair.

The claim is in line with the gathering pessimism over Scotland Yard's ability – or will – to bring anyone else to book. The EG government alleges that Mann, Sir Mark and others concocted their plot during meetings in London in late 2003 and early 2004. But a senior Home Office source said that, although the Metropolitan Police visited Mann at least three times in prison, and received relevant documents from the EG authorities, they were "essentially going through the motions". He added: "They are being asked to nail someone for perhaps planning an operation that in the end didn't take place. I don't see any will to ask any officer to push this too hard."

Legal experts now warn of the dangers of pinning a case on Sir Mark, partly because of an apparent lack of credible proof, but also because he has already been convicted of a similar offence on the same evidence. Although it remains illegal to conspire against another government while in the UK, no one has ever been prosecuted for the offence. The Met yesterday conceded that an investigation into the Mann affair was "ongoing", but refused to give further details, including the number of staff engaged in the operation.

Equatorial Guinea dropped its attempts to sue Mr Calil and Sir Mark – and British "security consultant" Greg Wales – after the case reached the House of Lords. Any chances of getting at the truth – in court or otherwise – appear to rest with Mann alone. The mercenary claims to have the material – including details of email exchanges with Mr Calil and Sir Mark, and bank details involving front companies in the Channel Islands and elsewhere, which allegedly detail the financial preparations for the operation.

Mann, who is preparing to speak to Met officers later this month, also has a powerful incentive for revenge against his former colleagues. "He feels incredibly strongly that these people could have come to his rescue, or at least helped his family while he was in captivity." Whether he has the ammunition, or the will, to extract revenge is quite another matter.