I've got a problem with the heater leaking warm air out through the floor ducts at my feet even when the whole system is switched off. Has anybody had similar problems and can somebody advise me as to how to rectify it?

Yeah I had a similar problem in datsun 1600 ute, I first noticed it when i put my foot though the floorboard. Some original photos have not been recovered yet.

check for leaks and split/broken hoses to the selector for the aircon/heater system. also there is a vacuum line to the plenum with a one way valve on it, check that that works correctly. there is a thread about the hoses etc called fixing the aircon or something like that so do a search. i put a post up on how to identify which hose is which when the labels have broken off.