Sir Mark Thatcher pleaded guilty Thursday to unwittingly helping to finance a foiled coup plot in oil-rich Equatorial Guinea in exchange for a $506,000 fine and suspended jail sentence. Thatcher, the son of former British Prime Minister Margaret Thatcher, admitted in the Cape High Court that he paid to charter a helicopter, which mercenaries planned to use in their attempted takeover. But he maintains he believed it was to be used for humanitarian purposes, according to his lawyers and a person close to the family.