Thanks Jared.

In both cases, the pipes run in wide chased out sections of the wall, probably about 8cm wide per pair of pipes. The pipes are held in pipe clips. Both of the walls that have pipes in them are going to be 'Dot & Dab' boarded with 12.5mm Plasterboard. I figured that if I dont actually fill the chase, then they will have room to expand/contract and the fact that they are boarded should prevent the risk of cracking.

Does this sound likely?