I know the true story, which is exciting enough, but Ritchie turned it into a violent, bloodthirsty tale as if the characters were super-violent and hell-bent on revenge. The whole harbor fight scene never occurred. Neither were there many Nastees on the island. They did not control it, nor had military camps, their own anti-aircraft, or even any overt soldiers. It was a Spanish territory. Ritchie took a few elements of the true tale, the party which was in fact thrown by a British agent on the island and to which 12 Italian officers and two German officers went, and the stealing of ships in the harbor. There was no intent to kill anyone at all or get in any kind of firefight. In fact, there were zero known casualties, 29 crew were captured. The only explosions were the anchor chains and, 30 minutes after the two tugboats with British commandos aboard arrived in the harbor, they were leaving the harbor with the three ships under tow, without any losses and with 29 D'Aosta crew as prisoners.
There were no German "S-boats" to chase them because there was no German naval base there and Germans were not in command.
The real characters weren't wild rogues in jail, but highly professional soldiers. Unfortunately, the main participants, March-Phillipps, Hayes, Appleyard, and Lassen, all died during the war.