Don't know what to tell you man. I have no clue what kind of bike your talking about.
-Anti
Go to Kelley's Blue Book online to get a rough estimate of what the bike is worth. I think its soley based in the United States, but I don't know for sure. Basically the Blue Book will tell you what the bike is worth depending on the mileage and age of the bike. Since I'm not very knowledgable about the UK I do not know if there is something parallel to this there. If not just convert km to miles and use the Kelley's Blue Book on the web. Then you can convert the dollar amount to pounds. Remember to try to talk them down some if they want more than the Blue Book pricing (unless they have some good modifications). I got my 97 for $350 below Blue Book. It's a good starting point anyway.
Sorry if that's not much help.
John L.
Since most of us are US based you might try the folks over on the UK specific GS500 website forum... www.gs500e.co.uk
http://www.gs500e.co.uk/forums/index.php
Otherwise, try looking at yoru local adverts to see what they usually sell for.