theres quite a good system on autotrader. you can type in make/model/year/plate and mileage and it gives you a what car price guide including how much you would get p/x or private etc. for free, there is a pay one but i dont see the point if you can get it free?
i'll try to find the link.
found it:
http://www.whatcar.com/newautotrader/va ... search.asp