I think its Autocar mag on line, which has a Parkers type guide, where all you do is input your details and they give an expected value. (may be What Car, Top Gear etc)
I know you said "online", but I usually have a free look at Parkers etc in the newsagent - you can get a few valuations from the different publications and take an average. Go with a pen and paper!
auto trader is used quite a lot within the trade its not a valueing site as such but it gives a fair indication of whats being offered and at what price. In fact some insurers use it as a guide when making offers to customers on total losses.
You can get a free valuation at http://www.parkers.co.uk/cars/prices/ for cars from 2002 to now. You only have to pay if you want to adjust the mileage etc.