Eliza Manningham-Buller
Specializing in counter-terrorism rather than MI5's usual counter-espionage, she was active at the time of the Lockerbie bombing by Libya in 1988. She worked for K-branch against the IRA. During the early 1980s she was said to be one of only five people who knew that Oleg Gordievsky, the deputy head of the KGB at the Soviet Embassy in London, was actually a double agent.