I replaced the microswitch, just to find that the original was OK. The reason I thought it was the switch was because I put a continuity tester across it and it was permanently on. However, when the wiring is disconnected from it, the switch functions normally.
With regard to the diaphram plunger, it seems to operate OK, moving out whenever a hot tap is switched on, and retracting again. I even operated the microswitch manually in time with the plunger, but to no avail.
I also tried disconnecting the DHW thermister to disable the DHW, and although the red LEDs reported the DHW fault, the CH side worked fine.
So it seems that, no matter whether the plunger is in or out, or the microswitch is off or on, there is always a call for HW.
Could this be a dodgy PCB, or have you any other ideas?
Many thanks.