It may seem a little bizarre that an American citizen was recruited to the KGB in London by Viktor Oshchenko in the late 1970s, but this story has never been revealed to the general public until recently.
This man was working as a salesman in a Hi-Fi shop in Tottenham Court Road, and he was easily led into an association with Viktor Oshchenko and effectively recruited as a KGB agent to work for the Russians.
Many months later he got scared and decided to tell the American Secret Service that he had been dealing with the Russians. The agreement was that he could remain anonymous if he told all and then acted as a double agent on behalf of the West. As a result at least one Russian KGB officer was expelled from Britain in the early 1980s.
You can read the actual transcripts of the American's debriefings (by the American Secret Service and MI5) on the USA Cryptome website here, and the Russian JAR2 website here.
The American managed to keep his identity secret when he appeared on behalf of the prosecution at my trial. He was known as Mr E at my trial, but he has also been referred to as "Linnell".