I actually solved this so I thought I'd post the answer, just in case anyone else gets the same problem.
I ran some hot (nearly boiling) into the washer tank & attempted to wash the front then the rear. After a few seconds the pump was once again audible for the rear & the front wipers stopped operating when they shouldn't have been.
The washers now operate perfectly, so I guess that the valve had jammed & that somehow this allowed the rear washer current to feed the front wiper motor. Odd but nevertheless it cured the problem so you have the answer if it happens to you too.