Sir Mark Thatcher arrived at court in South Africa this morning for a hearing in connection with charges that he bankrolled a failed coup plot in Equatorial Guinea.

The 51-year-old businessman, who is the only son of Lady Thatcher, the former British Prime Minister, was arrested at his home in Cape Town on August 25. He faces charges under South Africa's strict laws against mercenary activities.

In the current hearing, due to last today and tomorrow, Sir Mark is challenging a Government subpoena obliging him to submit to questioning by prosecutors from oil-rich Equatorial Guinea.

He strolled to the court with lawyers, smartly dressed in a suit and appearing relaxed and refused to speak to the media besieging the court.

"We are going to court to defend our client's right to remain silent," his lawyer Alan Bruce-Brand told the AFP news agency.

"It is contended by our legal team that this (questioning) is an inappropriate and improper procedure."

He added: "It's an interesting case and we are confident about our chances."

Sir Mark had been ordered to appear at Wynberg Magistrates' Court next month, after the South African Government granted an Equatorial Guinea request to question him about the allegations.

His lawyers argued, however, that it was unconstitutional to subpoena him to respond to those questions while he has related South African charges pending.

There was some surprise that Sir Mark had turned up at the hearing today, as he was not obliged to do so - and the case is being recorded on video to be shown on television, in a landmark trial for admitting cameras to the South African courts.

Today Peter Hodes, Sir Mark's barrister, told the court that to expect his client to answer questions in open court before he faced trial in South Africa on related charges was a violation of his right to silence.

"We are contending in this case that there is an infringement on the pre-trial right to silence ... and it constitutes an infringement to all the rights relating to a fair trial," said Mr Hodes.

He added that the justice system in Equatorial Guinea was not fair or impartial. He said: "This (South Africa's justice) minister and director general are assisting in a trial that does not even meet minimum international standards.

"What we are dealing with in Equatorial Guinea is a military tribunal... there is a military court there, and judges are not qualified."

Equatorial Guinea wants to question a number of prominent Britons about allegations they financed a plot to overthrow President Teodoro Obiang Nguema, who has ruled Africa's third-largest oil producer for the past 25 years.

The prosecuting authorities say they may seek to extradite Sir Mark to face charges. South African justice officials have said that they believe he could get a fair trial in Equatorial Guinea, but that no extradition request has yet been received.

Sir Mark has lived in South Africa since 1995. He is a friend and neighbour of Simon Mann, a former British special forces commander and soldier of fortune accused of masterminding the failed plot, who was sentenced last month to seven years in prison for trying to buy weapons from Zimbabwe's state arms manufacturer for use in the coup.

Mann's 67 accused co-conspirators, arrested when their aging Boeing 727 landed at the Harare International Airport on March 7, received sentences of 12-16 months for minor immigration and aviation violations.

Sir Mark was released after posting bail of £167,000, reportedly paid by his 78-year-old mother. His wife left South Africa in September, and since then he has been staying alone in their luxury home, obliged to report to the local police station several times a week and making few public appearances.

He is accused of helping to fund the purchase of a helicopter, breaching laws banning South African residents from taking part in foreign military action. His lawyers maintain that the funds were an investment in an air ambulance venture for west Africa. He could face 15 years in jail if convicted.

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