Have you started tiling? If not set the tiles out so that the socket does not fit in the middle of a tile. It's much easier to cut two adjacent tiles to fit rather than cutting out the middle of a tile.
Also it's much neater and easier to remove the socket (after removing the fuse at the consumer unit!!) and tile behind it. You may need longer retaining screws if you do this. You will need to cut a rectangular shape out of each tile. Use a template to calculate the size of the cut (a piece of thin card is ideal), mark the front of the tile in felt tip (NOT the back as sometimes felt tip is absorbed and shows through). Then scribe just outside the line. Put masking tape over the scribe lines. Use a tile saw (or an electric tile cutter) to cut down to the bottom line and then carefully nibble the waste away. Use a proper tile nibbler for this. It takes a while but you have to be patient or you could crack the tile. After checking it's the correct fit, 'butter' the back of the tile with adhesive
(not the wall) and after setting, replace the socket. Dont overtighten as this could raise the tile.