I think Macten is offering some good advice here. You can't hold off on works whilst you wait for the DIY cetre to pull there proverbial fingers out. I'd right a strong letter with some carefully chosen terminology like "not fit for purpose" and "consumer rights" and see if that prompts some action. If you have an experienced, qualified tiler doing your works as well get him to write a "report" of the failure and attach an invoice for the cost, inconvinience etc.
Keep us updated though.
Keep us updated though.