It cannot have been very satisfying work being a Czech spy in 1986. This was the fag end of the Cold War. In Moscow, a dynamic new boss, Mikhail Gorbachev, was opening his country up to the West, but the rulers of communist Czechoslovakia – as the country was then known – were intent on resisting any change.

Their country had experienced a glorious explosion of freedom during the Prague Spring of 1968, when a bloodless revolution appeared to have created a society uniquely combining personal liberty with a communist economy. That experiment was crushed by order of the authorities in Moscow, who put the dregs of the Czechoslovak Communist Party in power. These ageing mediocrities were still in office 18 years later.

Even as Gorbachev talked peace and rapprochement with Ronald Reagan and Margaret Thatcher, the bosses in Prague expected their foreign agents to continue the usual business of hunting for traitors and gleaning information that could be used to weaken the West.

Jeremy Corbyn’s passionate Hyde Park Stop The War Speech in 2003

Genuine traitors can be hard to find in a foreign country, or even to recognise when stumbled upon. The eastern bloc’s spy services must have been squirming with embarrassment over their failure to welcome Michael Bettaney, a drunken, mentally unstable British intelligence officer who approached them in 1984 with high grade information for sale. His behaviour was so odd that they turned him away. They learnt their mistake when the British put Bettaney on trial and sentenced him to 23 years in jail.

What could Lieutenant Jan Dymic, a Czech intelligence officer based in London in the wake of that fiasco, do to convince his bosses that he was earning his salary?

A quick study of the British press would have told him that there was an MP named Jeremy Corbyn, who believed that the UK should pull out of Nato and scrap its nuclear weapons, and who was prepared to meet people shunned by every other MP. Corbyn had caused uproar by inviting two convicted IRA volunteers into the Commons in December 1984, two months after an IRA bomb in Brighton had almost killed Margaret Thatcher.

So Lieutenant Dymic arranged to have tea in the Commons with Jeremy Corbyn, as documented in The Sun newspaper today. He listened to the Islington MP’s well-known opinions on British and American foreign policy, and reported back to his superiors, who may have been dense enough to think that their man in London had found a route to the heart of the British establishment.

From the British side, no harm was done. Jeremy Corbyn was never an intelligence asset. A genuinely dangerous spy is someone who looks trustworthy but has a secret double life, like Kim Philby. Jeremy Corbyn was always what he appeared to be. His office is stating an obvious truth when they say that he never had any privileged information to give away, because no one in the British establishment was going to share sensitive secrets with such an open left-wing radical.

But it is a very weak defence that some of his supporters have put up, that he could not have known that the “Czech diplomat” he willingly met was a spy. Among the countries of the old Warsaw Pact, Czechoslovakia’s intelligence service was known to be second only to the Soviet Union’s. Anyone who aspired to be prime minister one day should have known better than to hold a private meeting with anyone below the rank of ambassador from that embassy.