While the Germans jumped into the Atlantic, a 20-year-old David Balme and small team of sailors climbed into a rowing boat with simple instructions: Get what you can out of her.

Balme, who’d been in the Navy for seven years, could not believe the Germans “would have just abandoned this submarine” and was convinced U110 was either booby-trapped, or armed crewman were still on board, lying in wait.

Instead, the boarders found U110 deserted. Telegraphist Allen Long quickly located the coding device which looked like a typewriter. Long “pressed the keys and. finding results peculiar, sent it up the hatch”.

Balme’s party spent six hours salvaging what they could from U110, all the time compressed air hissed from broken pipes and the boat shook under the distant detonations of depth charges being dropped as the convoy escorts harried other suspected German submarines.

Bulldog tried to tow the crippled U110 to Iceland, but she foundered the following day. The destroyer continued on to Scapa Flow in the Orkneys, the RN’s main base in both world wars, where the ‘typewriter’ was handed over to an intelligence officer. “We have waited the whole war for one of these,” he gratefully thanked Balme and his shipmates.

The salvage operation – codenamed Primrose – was, the Admiralty ordered, “to be treated with the greatest secrecy and as few people allowed to know as possible.”

And so when George VI presented David Balme with the DSC for his part in the mission later in 1941, the monarch apologised that “for security reasons” the award could not be higher.

But he did tell the junior officer it was “perhaps the most important single event in the whole war at sea.”

The Enigma machine and accompanying codebook ended up at Bletchley Park, where they would be exploited by maths genius Alan Turing and his colleagues, allowing some German radio traffic to be read by British intelligence.

The story of the seizure of the machine by Balme and his shipmates was kept secret until the mid-1970s and ‘Hollywoodised’ in 2000 in the blockbuster U571; the fictionalised account has American submariners, not British destroyermen, rescuing Enigma from a crippled German boat.

David Balme’s career in the RN after Bulldog/U110 was no less dramatic; he commanded a detachment of gunners protecting a merchant ship on the Malta convoys (which was sunk), transferred to the Fleet Air Arm as an observer and flew missions in the Mediterranean; and was the youngest lieutenant commander in the RN when promoted to that rank.

After the war he worked in the family wool business in Hampshire.