Once again, thanks for your replies and for taking an interest in our issue. I could ask the mobile chap who did the last hab service in May 2022 if he thoroughly cleaned the fridge flue and the burner but I'm sure he'd say he did. I've no reason to doubt that but, in reality, who actually knows. Last weekend, we arrived at a motor sport event and parked in the camping field that is certainly anything but level. At that time, the fridge had been on over the previous night on mains and then on 12v for our journey and it was cold as it should be with the ice cubes frozen.
Parked the van as level as was possible but still with a tilt front to back. Fridge on to gas and next morning it had obviously not worked as it should and the ice cubes had turned back to water. Once home, fridge emptied and left turned off until Tuesday morning. The van sits pretty level on its hard standing at home and, as a test I fired it up on gas, filled the ice cube tray with water and waited. After maybe an hour or so ice was just starting to form in the tray and continued until all the tray was fully frozen. It's stayed that way until this morning when I've turned the fridge off and disposed of the ice cubes.
On the basis of that test, it appears that the fridge is working fine on gas as it does on the other power sources and the only variable is the front to back tilt of the van and, therefore, the fridge. I think it's time to consult the people at a local, newly opened, caravan/motorhome service centre who have recently carried out a small repair (*) that a local dealer and the chap who did the last hab service said they couldn't do the latter even doubting that it could be done without major dismantling.
FD
(*) Some might remember my struggle to replace a high level LED marker lamp to the front of the van where the connecting wire wouldn't let the lamp unit pull out so a replacement could be fitted. I'd taken it to dealers and talked to the mobile service man but all said they couldn't help. Visiting Code 11 Ltd in Ossett to look around shortly after they opened their new workshop I discussed the problem with one of their service people who said he'd done this job before and had a way of getting the unit/cable to pull out sufficiently for it to be replaced. Booked the van in, unit replaced and just charged for one hours labour (I had the replacement light unit already). No other connection with Code 11 Ltd other than a satisfied customer.