Hi everyone, thank you very much for all your helpful replies. They are making me think it through abit.
Yes, it is not putting out 240 anymore, it manages about 190 even when it is cool and newly switched on for the day. It used to run the fridge reasonably and the hoover at the same time...now it wont run the toaster (new item) so I have to switch it to inverter to make toast...
It does a very reasonable job of charging up batteries but if I run it for more than two hours (say the batteries have gone quite low)the charge drops very low even if the batteries have not finished charging.
It will start off around 45 amps and drop as low as 2 or 3.
It had some maintencance recently where the engineer tried to adjust it get the best out of it,
Cooling, it had a new radiator last year or year before. Not sure what I should clean every two years...am not very familiar with inner workings which are of an electrical nature, quite good with the basic end of things like changing the impeller, changing the oil, clearing out blockages in teh water flow etc.
The box thing, sounds like it is the GRPthing which stops water going back into the engine. It doesnt look exactly like the nice drawing sent to me (thank you) I reckon it might be a bit grubby in there after a few years of no maintenance tho so will have a look at it soon!
Sorry I didnt reply sooner, I dont expect immediate responses and am always thrilled to get some, but ususally wait a while to see what comes in...but I really do appreciate it. I have always managed to get to the bottom of it after everyone has given me ideas. Big thanks will post again when looked more closely...