Lo and behold our boiler started misbehaving too.
Central Heating fine but intermittent hot water - usually when I was in the shower covered in shampoo (stuff for hair, BM) and soap.
Phoned Gary the Tank on Thursday, he popped round and diagnosed a faulty relay on the PCB. Not good - a new PCB is about £140.
He phoned me early Friday to say that he found another (not working) PCB and that he'd come round to take out the duff relay and relpace it with the relay fro the one he found.
Took him half an hour last night. All sorted. Perfick.
Total cost ? Three pints - he wouldn't take any money.
Top geezer.