Profpointy
Legendary Member
There is some stainng on the low vertical grout lines that would indicate that there is some water ingress into the substrate, (mould growing in the porous plasterboard behind). Tile on tile is possible with correct prep & adhesives, personally I would not recommend it. Plasterboard is not a suitable substrate for tiling in a wet area (shower). I would strip back to the studwork, board the walls with tile backer board. Tape the joints & tank the walls. Prime with SBR (some adhesives may not require priming) then tile/grout.
Thanks, our plumber mentioned the grout staining suggesting it had been done wrong (like quite a lot in our house). Thanks to various advice here I think ripping the tiles off to check what's underneath, be it OK or shyte, is the best bet