Short answers, pulled from the story.
The Battle of Cape Matapan was a naval engagement fought from the 27th to the 29th of March 1941 between Allied forces (the Royal Navy and Royal Australian Navy) and the Royal Italian Navy in the eastern Mediterranean. It took place off Cape Matapan on the south-western coast of the Peloponnesian peninsula of Greece.
Cryptanalyst Mavis Batey at the Government Code and Cypher School (GC&CS) broke Italian naval Enigma for the first time in late March 1941. The decrypted messages, codenamed Ultra, revealed the Italian fleet's composition and sailing plans, allowing Admiral Cunningham to intercept and ambush the Italian force. Admiral Cunningham visited Bletchley Park personally after the battle to thank Dilly Knox and his team.
The Italians lost three heavy cruisers, Fiume, Pola, and Zara, along with two destroyers, Vittorio Alfieri and Giosuè Carducci. Fiume sank at 23:30 on the 28th of March; Zara was torpedoed by HMS Jervis at 02:40 on the 29th of March; and Pola was sunk by torpedoes from HMS Jervis and HMS Nubian shortly after 04:00.
Midshipman Prince Philip of Greece and Denmark served aboard HMS Valiant and was in command of searchlights that illuminated the Italian ships during the night action on the 28th-the 29th of March 1941.
Italy lost up to 2,303 sailors, most of them from the cruisers Zara and Fiume. The Allies rescued 1,015 survivors, and the Italian hospital ship Gradisca recovered another 160 after Admiral Cunningham broadcast the survivors' location and granted safe passage for Italian rescue operations.
The British kept the involvement of Bletchley Park and the Ultra decryptions classified for decades to protect the codebreaking programme. The underlying documents were not declassified until 1978. In the absence of the real explanation, alternative theories circulated, including a 1966 account by H. Montgomery Hyde alleging that a spy named Betty Thorpe had stolen an Italian codebook; Hyde was found guilty of libelling the dead. Italian official records were not corrected until Mavis Batey demonstrated Dilly Knox's decryption method in person to the Italian admiral overseeing naval history.