A confidential exchange of letters, released after 30 years of government secrecy, between former prime minister Margaret Thatcher and David Owen, then the leader of the the Social Democratic party (SDP), has shed new light on an enduring MI5 spy scandal.
The real-life espionage drama, which began in 1983 with a mysterious Moscow death and was followed by the arrest in Britain of a would-be MI5 mole, led to a full-scale internal investigation into the security service and fuelled an escalating diplomatic feud with the Kremlin.
