Thanks this is what I'll do if I can't find a personal recommendation. As it happens Simon Pollard looks after the engine/gearbox/electrics of my boat and I would recommend him wholeheartedly, provides a first class service, but sadly can't help with this problem.
Interesting, wasn't aware of this, thank you. The Eberspacher unit cycles on and off, once the boat has hot water at the taps, so I'm sure that it's something like this, as adjusting the thermostat (which has been replaced) makes no difference at all, other than cut the unit out when thermostat temp = cabin temp.