In reply to Fraser:
I bought in the area you are looking in about 18 months ago. At the time, houses were going fast and we paid about 13% over the o/o price. We assumed we were paying well over the odds but we really wanted the house and we could afford it. It in the event, it turned out that we only won by a couple of hundred pounds, so actually we were pretty much on the money.
The solicitor we used advised that we base our offer on the valuation price rather than the o/o figure. Our offer was the valuation price plus about £2000, so it seems that this was good advice. If the market is slower, or you aren't desperate for that particular property go slightly under the valuation. I don't know if it still holds though - as above talk to your solicitor as they will have a better idea what is going on now.