The wiring, I have to admit, is a bit hapharard. 20 year old boat and a few enthusiastic amateurs means it's all a bit bodged looking. The fuse was a 20A, and the spare, taped beside the main fuse in a case, was a 20 too. I've tried 25A ones (biggest in Halfords!), and no help.
I'll check the manual over the weekend for fuse size, and test the voltages you suggest. Had a good check over this evening for loose connections/corrosion etc, but cannot find anything.
Great part of the world to live, would love to start a dolphin spotting rib company, but the area is pretty well covered already