Hot pipe, blown plaster.

[parkerusm]

Hi everyone. Had a shower installed, pipes embedded in an old (1933) thin bathroom wall. The new plasterboard on the other side started to bulge outwards then cracked. I've removed it and you can see from the picture the wall is made of wood and some sort of clinker block. I'm guessing the wood expanded when the pipes heated, but am not sure.
I can fill and skim it but is there some way I can insulate it or leave an expansion gap so it doesn't happen again?
Thanks.

 

[Wayners]

Lightweight filler over. Just a thin skim then a powdered toupret over that.

 

[foxhole]

Looks to be timber at base ? If this got wet it would have expanded causing the damage.