Captain Who?

Watching the present series of Torchwood has reignited a little niggle which has irritated me ever since the programme first started. Why is the main character who goes by the name of Captain Jack Harkness wearing a WWII style greatcoat with a Group Captain's insignia on the shoulders? This - as far as I am aware - has never been explained. Is he (or more precisely, the man he took the name off) supposed to be - or have been- a Group Captain, in which case he should be addressed as such, or is he some other sort of Captain who has simply happened on the coat? And just to add to my irritation, I'm reasonably sure that when his character first appeared in a Doctor Who episode the insignia on his coat was that of a Wing Commander. Is this just a continuity boob, or what. As an ex-RAF National Serviceman this really grates!