Problem solved!! it turned out to be just the cable which needed attention. More than two weeks later Dometic finally came back to me with details of the company they use for service. In the meantime Cox Automotives gave me a price list which was actually more reasonable than we expected.
In the end a local marine electrician came and tested the system, the cable was found to have shorted, was replaced.
I would recommend Cox's for the speed of reply and we found the costs to be less than we had expected.
Many thanks to all who took the time to try and help us.