I have given this a 10 because it was interesting and gave a complete overview of the Cambridge spy network that was uncovered. No mention of a fifth man, or sixth, it just concentrated on the facts.
This is unlike the accompanying drama Spies Among Friends which I found overly long, drawn out and flawed by tokenism (the invention of characters).
Roger Allam is an excellent narrator and the historians and subject experts were interesting and informed.
The "old boys network" can never have had a worst moment than the covering up, or just ignoring, the misdemeanors of their chums. Asonishing that Antony Blunt was outed in the 1960s and it was hushed up. As he was the Queen's art curator at the time, this really didn't do her any favours in retrospect.
It took Maggie Thatcher to out him publicly in the 1980s.