Hi Craig..I'm a trading
standards approved business and yes it is true that you have to give the company the opportunity to rectify their mistakes. If they refuse, then you can get someone else in to put it right and deduct the costs from their payment. You may also be able to bill them for additional expenditure but I would take legal advice before taking that step. You can fit the sanitaryware temporarily and I would include this in the letter or a follow-up letter, so that they are aware they will have to remove the products before they can rectify their mistakes.
Also, I assume they gave you an estimate or quote for the works together with the scheduled start/finish dates. That document should also have specified the payment terms and if it says 'on completion of works' then you do not legally have to pay them anything until the works are completed.
Also, a little unknown law.. since 1 Oct 08, all estimate/quotes/contracts over £35 that are concluded during the visit, must have a cooling off period of at least seven days and must include details of the consumer's right to cancel the contract. It does not usually apply to contracts concluded after the trader has left the premises but your trading standards staff may be able to advise whether your contract should have had it included, if it was given to you after they had left your premises.
Good luck!