Originally Posted by tabetha

I may be wrong here, but I think your car is the one that has a water valve on the bulkhead, that are notorious for giving trouble, it may be that hot water is not actually getting to the heater to start with if this valve is duff.
It should be visible on the bulkhead somewhere with two water pipes going to/from it and a cable or wire connector from the heater control.
tabetha