If a tile is moving away from a wall 10or20mm, something is very wrong. Let me rephrase that, the tile adhesive has not bonded/stuck to the wall correctly. not really a big deal, unless you continue to use it. A professional fitted bathroom (tiles and sealant) should last over 10 years. Your rights with TW, i couldn't advise on, however, 2.5 years is nothing! Call them, tell them, as 2.5 years would not come under wear and tear. Oh, and stop using the shower, as water will be damaging the wall, as it will get in behind that loose tile. Its an easy fix, however, you must stop using the shower or it could be a real big issue....wall damage. And then you may find that WT says its your fault cuz you kept using the shower.