Exactly right there, under the consumer rights act 2015 you have to give the tradesperson the opportunity to rectify or replace work/goods that do not meet requirements.
As you have said, how an earth have they used Bal Greanstar in a moisture environment, i use Greenstar all the time but only in kitchensTake it down before it falls down. Thats really poor form. Greenstar is only suitable for ceramics less than 300x300. On porcelain it won't dry properly and won't have sufficient bond to the tile. Must use cement based. How thick are the tiles and what are they being fitted onto? If you say plaster I might cry.
Any pictures of what they have done?
Reply to the thread, titled "Adhesive advice and porcelain tiles" which is posted in Tile Adhesive and Grout Advice on Tilers Forums.