According to John le Carré, ‘The Spy and the Traitor’ is the best true spy story he has ever read. Its protagonist, Oleg Gordievsky, has lived, in a safe house, under MI5 care for 35 years: “a captive of history”. To this day, he cannot leave the house unaccompanied.

Over the course of an enthralling evening the book’s author, Ben MacIntyre, explained that he first met Gordievsky five years ago, whilst working as a reviewer for Times Books – and knew straightaway that this was a story that needed to be told in full. He has spent over 100 hours interviewing the former KGB agent, who has a “prodigious memory”, thanks to his KGB training.

McIntyre’s view is that spies don’t usually make a huge amount of difference. They don’t, he argues, usually change the strategic direction of the state for whom they work. Gordievsky is different. He had a profound effect on how both Margaret Thatcher and Ronald Reagan viewed the Soviet Union.

And yet, Gordievsky’s life nearly played out so very differently. He was a product of the KGB, having been brought up in a KGB compound, the son of a KGB officer. His father had taken part in the KGB purge of 1937 and was, according to his son, riddled with guilt – but unquestioning.

Gordievsky is academically excellent, not to mention a gifted linguist, and attended Moscow State University where he learned Swedish, Danish, Norwegian and German. This is where he had his first contact with the KGB’s recruitment arm. He took up long distance running, enjoying the solitude and, while on a university track team, befriended a Czech student, Standla Kapman, a trainee for the Czech Intelligence Service – who wasn’t (yet) a dissident, but who asked questions. The duo sat up night after night discussing Communism.

During his final year at university, Gordievsky was sent to East Germany where he witnessed, thousands of troops arriving to build the Berlin Wall. Whilst “profoundly shocked” by this evidence that the State was not what he believed it to be, that didn’t stop Gordievsky from joining the KGB and attending School 100, which he loved. There, he learned all of the arts of Russian espionage, including surveillance and counter-surveillance.

Gordievsky’s first job was at the infamous Lubyanka, in Department S, which created “illegals” – personas created by the KGB under false identities and implanted in other countries. His big career break came when he was sent as a “legal” to Copenhagen with his wife, Yelena, another KGB officer. They were astonished by the freedoms they found there.

In 1968 came a defining moment: Soviet tanks rolled into Czechoslovakia, to crush the Prague Spring rebellion. Gordievsky was – still is – furious and went to a bugged telephone in Copenhagen where he delivered a harangue to his wife about this criminal act. In doing so, he was delivering a message to the West.

Unfortunately, the West heard, but misunderstood. His second attempt to engage also failed – and, by now, he was under surveillance. The Danes had realised that Gordievsky was KGB and had spotted him buying porno mags, which they viewed as a potential blackmail opportunity (the KGB, for all its horrors, was notoriously prissy). An attempt to honey trap Gordievsky followed – and failed.

The years rolled on and Oleg Gordievsky transitioned from one country to another eventually, in 1975, ending up back in Copenhagen – where MI6 was waiting for him. And so began a one-year courtship – following which Gordievsky began revealing an astonishing amount of information. The stakes, commented MacIntyre, dryly, could not have been higher.

Gordievsky was able to identify who the KGB had recruited in the Scandinavian countries – although MI6 didn’t arrest them all at once; that would have given the game away. By now, he had fallen in love with a woman called Layla. Desperate to divorce his wife, but aware that such a move would damage his career (further KGB prissiness), Gordievsky told the British they had to find a way to smuggle him out.

Veronica Price, Gordievsky’s handler, decided that the only realistic way of doing this would be to abuse diplomatic privilege; in essence, bundle Oleg into the boot of a diplomatic car and drive him into Finland. However, they needed a signal from Gordievsky for Operation Pimlico to commence – a signal which took seven years to come.

In the meantime, Gordievsky was instructed to infiltrate the British section of the KGB and to learn to speak English. In 1982, he was given a posting in London, having memorised parts of the KGB archive – including its file on trade union leader Jack Jones, a fully paid-up Soviet spy. He had also memorised the file of Labour MP Ron Brown, who the KGB wanted to recruit (that plan failed because Gordievsky was unable to understand the politician’s accent).

The biggest KGB file by far was that of Michael Foot, That’s because from 1949-68 the Labour MP and one time Prime Minister hopeful had dozens of meetings in London with the KGB, who “paid him a lot of money” and fed him with information they wanted him to use. Although Foot did cut off contact after Prague Spring he was, says MacIntyre, “idiotic” to allow himself to be trapped – and blackmailed.

If you’re wondering why Foot never faced any consequences, it’s because when MI6 handed his file over to Sir Robert Armstrong, then cabinet secretary, that gentleman did nothing, hoping that Michael Foot would lose the 1983 election he was fighting – which he did.

Leila and their two daughters joined Gordievsky in London with no idea he was working for the British. The family loved life in the UK’s capital and Gordievsky’s career went from strength to strength; he was promoted four times, thanks to the British continually expelling his superiors.

In 1985, Gordievsky was summoned back to Moscow, with no reason given. This summons sparked a huge debate within MI6, many of whose members thought he should be allowed to stay in the UK. Others thought this was a golden opportunity for Gordievsky to uncover more intelligence – and he agreed.

As soon as the Gordievsky family was back in Moscow Oleg realised he had made a grave mistake. And yet, he wasn’t arrested, due to complacency on the part of the KGB and that organisation’s desire to capture him in the act of spying.  Although he was desperate to get his wife and daughters out of Russia, in the end he decided to escape the country on his own, believing it too dangerous for the whole family to attempt to do so at the same time. He also, tellingly, didn’t trust Leila not to tip off the KGB.

MacIntyre recounted Gordievsky’s escape in colourful detail, the entire auditorium hanging on his every word. It was the stuff that action movies are made of, this 80-second operation involving tanks, misled surveillance, a duel between diplomatic and KGB cars and strokes of good fortune. Somehow, Gordievsky made it to the West – and freedom.

That, of course, is not the end of the story. A “distraught and disbelieving” Leila was arrested and put under house arrest for six years; she and the couple’s two daughters were ostracised. The KGB were so furious that, despite personal appeals from Margaret Thatcher, they refused to let Leila join her husband, although they had no choice after the Berlin Wall came down. By then, the couple’s marriage was long-dead and their relationship ended in extreme acrimony; Gordievsky hasn’t seen Leila or their daughters for decades. He does, believes MacIntyre, regret that – but wouldn’t change what he did.

In fact, MacIntyre has never heard the former KGB colonel express any remorse for any of his actions. He set out to wreck the KGB and, while he couldn’t manage that, did cause it a huge amount of damage, of which feat he is proud.

These days, Gordievsky is “insouciant” about the KGB threat. This wasn’t always the case, MacIntyre assured us, but the former spy is aware that, should Putin try anything, it would be tantamount to an act of war. Today, Oleg Gordievsky has “lived with fear for so long that he is no longer frightened”.