The British mercenary Simon Mann today vowed to testify in court against Sir Mark Thatcher and the oil tycoon Eli Calil, the two men he alleges were co-conspirators in the failed attempt to take over Equatorial Guinea in 2004.

Speaking as he flew back to Britain from the west African state, Mann made clear he had no intention of drawing a line under the episode, and would welcome a fresh inquiry that could lead to him giving evidence against his two old friends.

Mann warned Thatcher and Calil, who deny any involvement in the plot, that he will repeat in a British court the allegations he has made to Metropolitan police detectives and in front of a judge in Equatorial Guinea that they were both key players.

"As far as I'm concerned, I am very anxious that Calil, Thatcher and one or two of the others should face justice," Mann said. "I am very happy to restate those things in court in the UK as a witness for the prosecution."

He added he was "happy we did not succeed [in the coup attempt] in 2004".

After receiving a pardon on Tuesday from President Teodoro Obiang, the Old Etonian former army officer touched down at Luton airport in a private Falcon 900 jet at 1.32pm yesterday with his brother Edward and sister Sarah.

He had served five and half years in jail, including just over a year of a 34-year jail sentence in Black Beach prison in Malabo, the country's capital. Mann, 57, was convicted of trying to take the oil-rich country by force as part of a coup plot in March 2004.

Detectives from Scotland Yard's counterterrorism command will interview Mann as part of an investigation into allegations that the failed plot was partly hatched in the UK.

A team of anti-terrorist officers have been investigating the claims since the end of Mann's trial in July last year. A small team of British detectives interviewed Mann in jail over three visits in 2008 in which Mann is understood to have co-operated fully.

A spokesman for Thatcher said he remained delighted at Mann's release, despite the possibility Mann could now testify against him. A court date might be some way off. Anti-terrorist officers are working closely with the Crown Prosecution Service in what they say is an active criminal inquiry, but a decision on charges is not thought to be imminent and no one has been arrested or interviewed under caution as part of the investigation.

Mann's homecoming at a cold Luton airport was a long way from the heavily guarded courtroom in a steamy equatorial city where he was sentenced in 2008.

His plane taxied to a private hanger where he was met, out of public view, by officials before leaving by a side door to an unknown destination, possibly his £6m mansion in the New Forest which he shares with his wife, Amanda.

In a statement, he said his return was "the most wonderful homecoming I could ever have imagined".

"There hasn't been a moment during the last five and a half years when I have not dreamt of one day being back in Britain with my family. I'm especially looking forward to meeting my son Arthur, who was born a few months after I left the country and whom consequently I have never seen.

"I am hugely grateful to President Obiang for releasing me. It's the best, best early Christmas present my family and I could ever have imagined. As I know you will understand I have been away for five and a half tough years, much of it spent in solitary confinement. I now need time to adjust and so I would ask that you respect my privacy and that of my family during this period."

Mann has said he "felt like a guest … and not like a prisoner" at Black Beach and there were reports he dined on wine, chicken, steak and vegetables brought in from a nearby restaurant.

Niek du Toit, a South African who was also convicted of involvement in the coup attempt and was released alongside Mann with three others, claimed Mann was critical to the plot. "If you get to Mann, you get to everything," he told the South African Star newspaper, before flying back to South Africa.

"He was the key to everything … the only time I met any big players was when I met Thatcher, and that was when I met him for two hours to talk about helicopters he wanted for his business in Sudan. Only when we got to Equatorial Guinea did I know exactly what his business opportunities were."

Last night there was fresh speculation about why the plotters have been released now. Du Toit said it was to coincide with the visit by the South African president, Jacob Zuma, and he had been told, "Zuma and his government were involved in the negotiations for our release".

The four mercenaries who were released on Tuesday have been told they can never return to Equatorial Guinea.

Henry Bellingham, the Conservative shadow justice minister and a friend of Mann who campaigned for his release, said the Equatorial Guinea government appeared keen to rebuild international relations in order to secure markets for its reserves of oil and gas and that "Simon Mann was becoming an embarrassment to the government".

Mann's return to the UK is likely to spark a bidding war for his story. Besides newspaper interview rights, publishers said Mann's memoir, which is likely to read in parts like Frederick Forsyth's novel The Dogs of War, could attract an advance of up £250,000. One cautioned that to attract such a sum Mann must reinvent himself in the public eye as an "officer class Andy McNab" to attract readers who may otherwise be squeamish about his mercenary past.

Sir Mark Thatcher: The unanswered questions

The meeting in London

Mann has claimed he met Thatcher and Lebanese-born oil tycoon Ely Calil at Calil's London home in May 2003 prior to the coup. This meeting is likely to be central to any potential prosecution by British authorities. Thatcher could explain what happened at this meeting and whether the coup was discussed, as Mann claims. Calil has strongly denied being a coup plotter.

The money

Thatcher said he thought the money he provided to Mann was to be used to buy an air ambulance rather than for a coup. Mann has said Thatcher knew what his funds were being used for and that Thatcher was "part of the management team". Mann has also said he was at a meeting with Thatcher where they discussed how to run Equitorial Guinea after the coup.

The middleman

What was Thatcher's relationship with Niek Du Toit, a convicted co-conspirator? South African Du Toit said he met Thatcher in South Africa before the failed coup attempt to discuss the sale of helicopters for use in Sudan, where Du Toit said Thatcher had interests.

Robert Booth