That site looks to be a rough guide for insurance purposes but the prices look reasonably fair to me. Dealer prices are negotiable though so what you see on the screen will be less than you pay if you haggle.
The only guide I know of based on real sales is Glasses guide but that is trade only and other than dealers and insurance companies it is not available. You can always challenge a dealer to show it you to prove his price is fair and if he will not draw your own conclusions.
Prices will fall soon as we go into Autumn, Winter as sales drop and dealers want to clear their sites for new models and avoid having too many second hand ones hurting cash flow so haggle hard.