Thanks for the input guys.
Avi8tor, "It has NOTHING to do with the parcel of air question and everything to do with inertia being GROUND speed dependant. Ergo, extremely high rates of turn are possible." I think you've got it wrong there, imagine the parcel of air is the air inside a railway carriage, as the train trundles along at a constant speed you could fly a paper aeroplane around inside it, and it would behave no differently than it would if the train were stationary.
john_tullamarine, I phoned the RAeS library but they didn't have any articles indexed under 'downwind turns', do you have any more details?
Mark