I've discussed Christmas traditions within the Royal Navy in WWII here, though I didn't discuss ships engaged in combat. In general, ships that were engaged in combat on Christmas Day put off celebrating it until they were in safer waters. However, your Grandfather would have celebrated Christmas 1943 on the 25th December - Matchless did not engage Scharnhorst until the evening of the 26th. If he hadn't, he would have celebrated it in Murmansk either before setting off with Convoy RA 55A, or after the battle when Matchless returned to refuel.