He served aboard HMS Cassandra during Operation Acumen, which was the suppression of submarine attacks on convoys in the Atlantic supplying the USSR.
It was attacked by U-boats and torpedo bomber aircraft, and while no merchant ships were lost or damaged, the Cassandra was torpedoed by U-boat U-365 under the command of Oberleutnant zur See Diether Todenhagen at 0600 on 11 December when about 165 miles north of the Kola Inlet, to which she returned, escorted by the 20th Escort Group, under tow.
She was brought alongside at Rosta for temporary repairs and the cutting away of the damaged portion, but owing to delays, and especially the shortage of steel plate locally, it was not until 14 June 1945 that she was able to leave Kola for Rosyth, arriving on the 18th under the escort of HMS Onslaught.
After retiring from the navy, Leslie became the domestic bursar and fellow of St Edmund Hall, Oxford 1970-1988[23][24] In 1945 he was awarded the OBE.
He was awarded the Companion of the Bath (CB) in 1970 on promotion to the rank of Rear Admiral[28][29] He was married in 1953 to Margaret Rose Leslie, and they had a son and three daughters.