Trivia: Instead of the Royal Air Force, the series has a fictitious British Air Force with largely invented insignia. Ranks are taken from the RAF, but sometimes different rank insignia is worn (e.g. an air vice-marshal wearing air marshal's insignia and a flight lieutenant wearing flying officer's insignia, although another flight lieutenant wears correct insignia).
Plot hole: Burke is murdered so that Doward will be sent out to replace him, as both hold the same rate of CPO sonar operator. But there are many CPO sonar operators in the Royal Navy. How can the GRU be so sure that Doward will be the man chosen to replace him?