You could look to see what comparable properties actually sold for earlier this year through something like Upmystreet.
Thing is good houses are actually going for asking price...and some more here in Bucks due to severe shortage. Sealed bids are all the rage! However not-so-good properties/areas/new builds you can knock off almost 30% of asking price. It depends on so much more than national statistics.
Walton and Weybridge haven't dropped as dramatically as many places; we've been looking there for months, with up to £800k to spend, and it's obvious that whilst prices are down, they haven't dropped nearly as much as where we are now (Fulham). I think overall they are down about 15% in Surrey on the year, but that will vary as to type of property and location.
If it's near a good school (Cleves?) then the price will always be strong as there will always be demand, and the stock on the market is really low right now.
You could try www.homes.co.uk/guides/ which charts price changes by postcode and property type. I believe it uses land registry data, meaning the last 18 months have provided only limited figures, so a single high or low value sale can shift the average figures significantly. They do have data up to end May, though.