Hi everyone
Thanks again for all your wonderful replies. You convinced me that what we had wasn't normal and that I should push for a solution.
I asked the builder to come and have a look at the shower after two months (!) of not using it. He brought a moisture meter and discovered that the dark patches (there were still some) were, in fact, still wet. I also pointed out the very slight cracks in some of the grout. He agreed to consult with BAL again, and then decided to scrape out all the grout in the shower tray area and flood the joints with a liquid adhesive, so that it would flow into any gaps under the tiles.
After another 2 or 3 weeks he then got his tiler to grout with Mapei epoxy (medium grey). I must say this looks fabulous: not really any different in colour or appearance to the Bal
cement grout when dry, and of course stays exactly the same colour when wet. I really wish we'd had this in the first place!
We have now been using the shower for a couple of weeks without any problems: no cracking, and still looks perfect. Only time will tell whether it can stand up to the rigours of movement on a suspended timber floor (although there is not a lot of movement currently), but hopefully given that it is fine so far, this bodes well...
Of course I will keep you posted if there is any change, but hopefully this thread will be of some use to others experiencing similar problems in the future. In my opinion it really makes sense to use a fully waterproof grout like epoxy in a tiled shower tray, but of course only after making sure that the tiles have been bedded down properly in the first place.
Thanks again,
Paula