22mm gap to fill.

[merlecollins]

Strip all the loose back to the blockwork and fill in with Keraflex maxi which will fill in the shallow areas more effectively than sand and cement, but also go right up to your 22mm thickness with no problems.

Let that dry and fix with your rapid set

 

[Sean Kelly]

Yeah that's what I was about to say.

If the plaster came away so easily then I would hazard a guess some more is blown. Does any of it sound hollow? They look like fair sized tiles so wouldn't want them coming down on you whilst relaxing in the bath. I know you won't want to take more down and I could understand the customer not wanting to redo the whole job with another tile but is it safe?

I’ve scraped away all the loose bits and the hollow bits. All that is left is rock solid. However, these tiles are big 600x300. The tile to the right of the picture does sound hollow, but I can’t move it because it is hollow on the left and solid on the right. I got my trowel behind it but I’m pretty sure it would snap in two if I applied any more pressure. I have to leave it there because there are no spares. I plan to drop some addy behind it to make it a bit more secure.