Hi NickS, that is a very nice generator, if I lived in the UK I would pay you 750 pounds for it as it is. Serviced and load tested with wheels I might pay twice that. I recently bought an untested Lister/Petter twin cylinder generator on a trailer for 1700 Australian dollars, it`s in storage waiting for me to find the time to build it a small shed.
If you measure the wheel stud pattern you can probably get some suitable wheels from a local scrapyard.
Is there an hours meter on the control panel? The value is very much dependant on the amount of work it has done, the one I bought only had 95 hours on it! (due for it`s first oil change).
There is a UK based Lister engine collectors group on Facebook, consider contacting them if you are going to sell.
Good luck,
Bob