I would never use it in a million years, however rubbish it is though i doubt you will get them to agree its not fit for purpose, I have seen it used by a family member a couple of years ago, it stated on the tub that it could not be grouted until completley set, the instructions were followed and to be fair the tiles are still down today no problems.
The issue with the stuff is that it has to be used in a very specific way, ie not thickly, only with ceramic and must be left to dry out thouroughly before grouting.
The tiler shouldnt have touched the stuff with a bargepole, and to be honest at nearly £20 a tub, the tiler should have been able to get quality gear for you for less cash.
How long was it left before grouting?
PVA underneath,
tile and grout ontop will have left nowhere for the liquid to evaporate away allowing the adhesive to cure.
Horrible stuff and I hope you get a good result from B&Q but sorry this isnt what you want to hear, I cant see them stating that its not fit for purpose as a floor adhesive, as rubbish as it is.