Thanks Andy - but I have already tried turning teh fridge over daily for a week. The heating element is different for mains (works best) gas (works next best) and 12v (hardly at all - and at about 8 amps very bad for batteries). The pipework and heat exchanger is common.

The fault is usually caused by running out of vertical, and that makes stuff in the refrigerant fluid crystallise - and according to the chemists that is irreversible!

The remedy is a heart-lung transplant for the fridge at a cost of about 200squid.

But I might try it one more time, although of course since in the cold weather it does refrigerate I will have no way to test until too late....