christhelawyer : HiWelcome to JustAnswer. My first response will follow shortly. Please feel free to follow up if anything is not clear
christhelawyer : The builder is normally the person who ensures you have the Code Compliance Certificate, although you will find the council fees are added to his bill, unless this is a fixed sum contract.
christhelawyer : but there could be a limitation issue here. They should have checked at the time, and you cannot bring a claim more than 6 years after the problem arose. Here the problem arose when they bought in 2004 without code compliance. They could have asked you to complete this, but now would have difficulty claiming.
christhelawyer : Possibly you may feel some moral duty to help them, but from the facts you describe, they may not be able to force you to contribute.
Morole obligation vs have to. hmmmm.
christhelawyer : Perhaps splitting the cost of the council fees on a without prejudice basis, and without conceding liability may be a start.
christhelawyer : i suggest if they ask for more you would need to have a discussion with your lawyer, with all the information to hand.