In return, the Soviets expelled 25 British diplomats – all of whom were not spies. This was a brilliant move which caught the British government completely on the hop. Such a high percentage of our “diplomats” in Moscow were spies, that it was a practical impossibility to accidentally expel 25 who were not. In other words the Soviets had just informed us that they knew exactly who our spies in Russia were. That sent such a juddering shock though the FCO it even reached this bewildered new entrant. Secondly, spies of course do nothing useful or practical, and expelling 25 actual diplomats was a much more crippling blow to the work of the Embassy. The FCO does not have lots of Russian speakers standing around doing nothing, ready to step in and replace.
I don’t claim any great reason for retelling this, except it is interesting and I have never seen it published elsewhere.