Try this:
http://networkq.co.uk/nq/buyers/valuecar.do
It gives you trade prices, but not retail unfortunately. Its Glass's though which is what all traders use.
Dont use Parkers, 99% of the time its wrong! Dealers use Glass's and CAP, as mentioned above.
I agree Parkers seems out of date.
When I part ex'd my Celica for the Cupra I paid the £3.00 for the Glass's valuation which was spot on for what I'd been offered as a part ex price.