Well .. i have the cover off .. unfortunately the only diagnostic I can do is to test the integrity of the windings. I can remove the choke, RFI filter, AVR and diode bridge. I guess I can test the diode bridge. The brushes and sliprings are at the other end and would require complete removal of the generator. A pain since the generator is only mounted at one end and relies on the coupling to the engine to hold the other end up.

After thinking I realised the 240v generator is entirely independent of the 12v system so there's no point uncoupling the 12v alternator.

AHA. It's an MD1C AVR inside the cover plate visible now after cleaning. I had exhaust problems and there is soot everywhere. It's even possible soot = carbon = conductor is the problem and a good clean will fix it Luckily i just bought a nozzle fitting for my scuba gear to create an air jet. Well let you know. Thanks again for all your help!

OK, its and SL105G, 7KVa. Needed a mirror to see the plate (the 'front' is backed onto a bulkhead). There is no visible external AVR, choke, RF filter etc. Nothing. There is an on/off switch in a plastic box mounted on the side. From the diagram the AVR is inside the end cover plate which is easily accessible but I can't see anything inside there because it is so dirty. Will clean with compressed air next, then check the safety precautions for removing cover plate.