Oleg Penkovsky, a colonel in Soviet military intelligence, is in the record books for his spying. His information – passed to MI6 and the CIA in the early 1960s – helped President Kennedy manage the Cuban missile crisis successfully by identifying the extent of Soviet missile capability and how far the Soviet leader Nikita Kruschev was likely to push events.

The most useful strategic intelligence comes from penetrating the leadership of your enemy so that you understand not just their military capability but their intentions. That was something MI6 only managed late in the Cold War largely thanks to KGB Colonel Oleg Gordievsky, who spent a decade towards the end of the Cold War supplying intelligence to MI6 which revealed how paranoid the Soviet leadership was of a first nuclear strike by NATO. The British service could not initially believe it, but they eventually believed it.