I learnt about the history of our National Flag from a Royal Navy Commodore during my Cadet PO Instructors course at Whale Island - the home of naval drill and gunnery training - so obviously he was biased. It would appear that due to yet more meddling in our national affairs by parliament ( albeit a long time ago) that this is the only instance where the terms flag & jack are BOTH correct.
( In true Orwellian tradition I maintain that one is more correct than the other)
