The seller has misrepresented the property by failing to answer the solicitors enquiry as honestly and as a result of that misrepresentation you are going to suffer loss. The loss is obviously the cost of putting this right which is (estimate) £5000.
Any correspondence in respect of this is going to be better coming from a solicitor. The solicitor should write to the seller advising them of the findings, pointing to the reply in the Seller’s Property Information Form and the survey and asking for payment of the sun.
If the seller doesn’t pay then there is no alternative but to take him to court for the losses suffered as a result of his misrepresentation.
Can I clarify anything for you?