Kev wrote:
I have made a deep chasing into a wall to take 2 copper pipes for CH.
If I fill this with expanded foam will plaster stick to the foam. I
had thought about putting plaster board over the void and filling with
foam as a backing for support but I might be able to dispense with the
PB.

Kevin

plaster will stick to expanded foam, but it's a pain in the arse, if the
chasing is deep enough, cover the two pipes with fibreglass insulation and
plaster over - the pipes will have room to expand and contract and the
plastering will be easier.
Failing that, it's ex-foam *then* strips of plasterboard set into it, then
plaster.