passenger seat floor on left side front & back very wet, no sign of any dripping water, but found left drain below windscreen under bonnet to be blocked with leaves etc with about an inch of standing water sitting on it. Could this have caused the problem? In prcess of trying to dry out car.
Yes, steam clean around that scuttle panel area - especially in autumn/winter.
There may be a slight chance that the heater matrix is leaking into the car, but if you say that the drain was blocked, then it's most likely that.