Well, like many of the faults I have had, many viewed, no-one made any suggestions, and I've solved it myself - but I still am willing to share my findings with you all.
I had originally checked the fuses and they seemed fine. However, when I removed the fuse that controls the heater water valve mechanism, and replaced it (even though it looked fine) - the system started working again - so my recommendation is to check fuses by changing them - not by just looking at them.
Maybe that's common sense, but there ya go!