Hang on, notanan you are confusing reinstatement cost with the surveyors value of the house. Those are two entirely different things.
House price = the price you have agreed to pay for the house, say, £500k
Survey price = what the surveyor says the house is worth compared with similar houses in the immediate area and its condition etc, say £475K
Reinstatement price The cost of rebuilding the house exactly as it is now, say £350k, or for a listed house £600k. This price will determine the level of buildings insurance your lender will insist you take out.