We’re walking through a park in London where a Soviet spy once carried out a secret drop – just before he saved the world. ‘Most people have no idea how close we were to nuclear war at that time,’ says Ben Macintyre, author of The Spy And The Traitor, which tells the extraordinary true story of a KGB agent turned British informant called Oleg Gordievsky. ‘He was able to crack open the inner secrets of the Kremlin. No spy had ever done that for Britain before.’

Gordievsky worked undercover for the KGB – the Soviet secret service – in London in the early Eighties, sending reports back to Moscow. But he was also, bravely, spying for the West. ‘If Oleg had been caught he would have been tortured and executed, and most of his family would have been rounded up as well.’

Then came Able Archer 83, a Nato war-game training exercise in November 1983, leading up to a simulated nuclear attack. The Soviets thought it was real. ‘Ronald Reagan’s rhetoric about the Soviet Union as an “evil empire” was interpreted in Moscow as a direct threat. The Kremlin genuinely believed the West was going to launch a first nuclear strike.’