Six years on, it turns out that British secret agents in Moscow really did use a plastic rock in a park to spy on Russia in 2006, despite attempts by the government to dismiss the story at the time. We should not be surprised. Preposterous as it seems, the more improbable the charge, the more likely it is to be true in the sometimes schoolboy world of spying.
The James Bond-style bug consisted of a transmitter in a fake rock; as they passed by, Russian double-agents could download messages from palm-held computers. British spies could collect them in the same way. It was a hi-tech variation on the "dead letter drop". The problem occurred when the technology failed.
Covert operatives from the British embassy, little knowing that the FSB, successor to the KGB, had the "rock" under surveillance, were filmed walking past it. One was filmed giving sly kicks to see if a bit of old technology, the boot, could jolt the sophisticated device into working again. The clip was shown on Russian television, as was one of another British diplomat picking up the rock and walking off with it.
But if there is something absurd about this episode, so there is too about the fact that, according to MI5, Russia has as many agents based in Britain now as at the height of the Cold War. And presumably the West reciprocates, though Russia is far from the primary threat it was in the days of the nuclear stand-off.
All this has consequences. In 2006, the exposure of Britain's secret agents in Moscow was used by then President Putin to reinforce his campaign for new legislation against the foreign funding of Russian NGOs, and specifically those dealing with human rights and democracy. It also fed into a general deterioration in relations between Moscow and the West.
Now the climate is improving, the carry-on continues, with Nikolai Kovalyov, a former FSB head and now a Russian MP, interpreting the admission as a subtle attempt from London at a diplomatic rapprochement – even though it came in an interview recorded a while back for a new BBC documentary. If this is how Russia wants to see it, that does no harm. But it is high time that schoolboy spies, on both sides, grew up.