Thanks to everyone for their very helpful comments. I contacted BAL and was given similar advice,very speedily and efficiently, so top marks to them. Interestingly, whilst not saying as much, the inference was that the tiles might hold, but in what was described as a picture frame effect. I am going to ask the tiler to remove a few tiles in the shower area, which is the area of most concern to me to see if the adhesive in the centre has gone off or is still tacky and then take it from there. Another point made by BAL was that they advise against tiling on bonding plaster, but no explanation as to why.