House sold (contracts exchanged) in 6 weeks or so via estate agent (after trying not very hard privately for 8) for about 1% less than the asking price. Probably could have got the asking price by holding on but this was a simple chain so would have been dumb not to go for it.....so the answer to extrapolating an HPC from your one single instance of a long sale at low price is .............location, location, location.
IMHO the agents earned their comission, did a good job marketing it, suggested a realistic price. FWIW this price is roughly what I'd have expected it to be sold for 1-2 years or so ago, eg i think prices in this area (mid southern england coast) have been flat for that period.