I suppose technically he is right but an auctioneer selling a development should know the Stamp Duty situation for all of the properties. That's what annoys me about auctioneers they take all the benefits and none of the risks of buying and selling!
Complain to the Law Society, complain to the IAVA. Ignorance of the law is no defence but someone somewhere should ahve told you if there was or was not a Floor area cert. Solicitor wouldn't know until after booking deposit paid but before contracts signed.
Happened to me once in reverse, told client no FAC as there wasn't one with the contracts, turned out there should have been. Client pleased to save 20k oddly enough.