Thanks for the replies.
The similarity to Honda engines is promising as there is a Honda agent with a workshop in Bournemouth.
Although I said that I was seeking a servicing agent, I used the term to cover repair as well. In fact it is more than serving that I require. (I have also carried out that myself). Despite a change of spark plug, it is running rough and only after a long run will it take any significant load. It seems to be running far too rich as the plug is prone to sooting, and there is fuel escaping or being coughed around, sufficient to cause strong petrol smells in the living accommodation.
I've checked the air filter, with difficulty and that's as clean as new.
It would be probably a relatively easy fix if the generator could be removed, but there is sod all room to get at the top, and although it's designed to slide out, the exhaust exiting downwards prevent this. So whoever takes on the task, will have to know in advance what is being faced.
Since I only live just outside Blandford, I think I'll call round to Dometic at the brewery and somewhat demand support. I don't expect to get anywhere however.