if the walls were try when tiles were applied it sounds to me like the adhesive is not strong enough for the size of tile
Go to wickes - i have been using there adhesive for years and had no problems.
wickes paint is fantastic, it is as good as dulux and alot cheaper - although years ago there gloss was not very good, i was persuaded to try it as they get it from a new supplier (i tried there gloss one coat) and it was the best i have every used.
I always use wickes paint now - saved a fortune