replacing a single tile

I would glue some battens to the back of the plasterboard, overlapping around the edge of the hole. (Normally I screw them through the existing plasterboard, but you can't due to the tiles.) Allow a good area of overlap with both existing and new plasterboard so there's a good sized area for the glue to bond.

When they're stuck firm, cut a piece of plasterboard the right size and thickness to fit in the hole, and glue this to the battens (or screw if you think you can without breaking the existing glue bond).

Now tile and grout onto the board.

If the existing board is aquapanel (or some other cement based waterproof plasterboard replacement), then you might want to use similar. In any case, make sure the grout is well waterproof.

Instead of timber battens I normally use pieces of plasterboard glued to the back of the original. This will eliminate any potential problems with shrinkage in the timber, as well as using up scraps of PB.

Use some sort of tourniquet arrangement I suppose. Or use dabs of 2 part filler as the adhesive. You'd have to keep pressure on the batten for 3 or 4 minutes while it went off, but it bonds well to PB IME.

Allow a good area of

Something like that. I think I'd make it such that future access is easyly available, that left hand pipe looks a bit stressed to me...

I'd probably have 4 blocks on the wall and a tile mounted on a bit of WBP ply then use stainless mirror screws, with a blob of silicone under the heads, to attach that to the blocks finallly grouting round to waterproof the join. The grout will be easy to remove should you need access later.

Thanks all, I should be able to sort this now. Some good ideas.

Buy or make this Schluter Rema tile hatch:

Innovative! Magnetic cupboard catches are cheap enough. I suppose velcro would work as well

Get a fridge magnet off the fridge, cut it into four and glue it to the tile. Screw four steel brackets inside the hole. The grout will stop the tile falling out.

Pack of 10 magnetic catches for £1.73 and you want me to cut up a fridge magnet? :-)