Well the whole lot came to a deathly silence last night; the unit cuts off with the FP1 error everytime I restart now. I've given up. Luckily I booked a service call for Monday morning before it gave up the ghost. Of course, it's cold outside but I have great things to say about our gas fire insert. It is keeping the house from getting too cold. Yes, the JW3 jumper is cut - the installer cut it 2 years ago when we had these problems before. The compressor ran continuously for the rest of that year, but it warmed up shortly after that and we didnt realise that it continued to struggle. Ah so the DIP switch just runs the HWG pump. Well I'm impressed because the HSW was disabled and the pretank was definitely very hot at the top. The DIP switch was set to ON which according to the manual means that the HWG is disabled. Are there any important questions I should ask the HVAC guy when he comes on Monday?