milly
in answer to your question the correct way to do this would have been a minimum of 15mm over laid wbp ply with a movement joint where the different subsrates met, the boards would have required priming to topside and underside
ufh bedded in addy or self leveling compound , the tiles fixed on a serrated bed of adhesive to the floor and each
tile back skimmed to ensure a 100% solid bed fixing
as for the legal side
yes you do want recompense, but the full cost of ripping out and redoing not just your money back, im in the process of suing someone and its not as straight forward as it was, trading
standards are right and have the letter templates on their site, but if these get no result then seek advice from your solicitoras you now have to follow a certain path into court and give him every oppurtunity to respond and rectify