In reply to 3leggeddog:
The trouble with asking prices is that they often reflect too many uncertainties not least of which is unrealistic expectations and greed. Vendors tend to look at other asking prices in trying to establish theirs, and are less likely to take account of actual sale prices (such data can be more difficult to obtain and is often out of date in a changing market by the time it gets published). Surveyors will have better access to information on other sale prices (and the condition of those properties) than the general public.
Estate agents can be just as bad because they are in competition with other agents to get the property listed on their books, so there is temptation to give the vendor an optimistic figure which is what the vendor wants to hear. Once they have got themselves signed up with an agency contract, they will then start trying to chip away at the asking price to increase their chances of getting a sale. Immoral I know, but it happens a lot, because an agent who comes in with realistic advice at the outset generally won't get instructed because he/she is telling the vendor an unpalatable truth.
Then there is the market itself which can be rising (not at the moment), static (probably not at the moment) or falling (very likely at the moment) which necessitates constant monitoring. At the moment too high an asking price will not only not bring any response, it's likely to put purchasers from even viewing.
Extraordinary things happen in a falling market very much dictated by the personal circumstances of the vendor and purchaser, so it would be misleading to put any percentage on the drop from asking price to sale price. I've experienced cases where a purchaser intent on particular criteria which the property offers, offer the full asking price with the knowledge that it's too much so as to secure the property, and on the other hand I've seen an offer of £295,000 accepted on a house being marketed at £400,000 because the vendor has become desperate and HAS to sell.
Valuation in a falling market can be a nightmare.