Forgot to mention the actual fault. I disassembled the old heat pump, and found it was split into a pump (logically enough) and a separate part containing a flexible heating element wrapped around a grounded metal tube through which the water flowed (so no scaling problem). There was a larger metal tube wrapped around this - again, grounded. 5mm gap between the element and the shroud, and clear evidence of a large spark across them which had burned a hole in the element. No residue left, but I assume a spider or something had crawled through the gap. The only puzzling bit is that the flexible element itself should be insulated on the surface, so how did the spider short things in the first place.