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Provided you have not deliberately concealed matters/lied to the Buyers about the sate and condition of the floor (which you have not), you have absolutely nothing to worry about.
A Buyer purchases a property in its existing condition, and is under a duty to satisfy themselves as to the condition of the property, before exchanging Contracts. A Seller is under no obligation to disclose the state and condition of a property to a Buyer, who must rely on their own Survey. Therefore, even if you knew (which you didn't) that the floor was rotten, you would not have been under any duty to disclose this tot he Buyer, unless they had specifically asked the question in correspondence via their Solicitors.
The Buyers Solicitors are therefore just trying it on with you. Their only potential claim is against the Surveyor who carried out the Survey!
I hope this assists you and answers your question.
Thank you for your reply.
Thank you for your help. I don't have any additional questions at the moment.