I am quite amazed at some of the comments to the OP. Especially considering that nearly everyone posts in favour of the builder when a buyer wants to pull out of a new build and has signed contracts a year ago etc etc etc.
OP- If what you say is the case and you have both signed the contract and the contract is not subject to survey and the buyer is demanding a reduction blah blah blah, I would just refuse. If he refuses to close the contract hold on to the deposit and forget about the purchaser, because, if as you say, all is above board it sounds like he is chancing his arm.
I would also take some of the other advise and find out why there is a difference between he engineers and surveyors report. If there is something wrong with your central heating, and the contract is not subject to survey, the purchaser should never have signed the contract until his survey results were in.
Just my two cents, Im sure some people will disagree.