Zipadee Doodar wrote:
I have 2 irregularly shaped holes.

(stands back for funnies......)

One is in the ceiling where 3 redundant heating pipes go up into the
void, presumable to header tank etc. The hole is about 6 inches at
its widest, but irregular in shape. Pipes are still in place - wild
horses won't drag me up there. How can I fill it? Plaster just drops
out.

Second hole is where the skirting orta be, but pipes from combi enter
it horizontally to disappear under the bath next door.

Would expanding foam be ok, since it is impossible to do anything
neatly, owing to pipes, location etc. I assume it would not buckle
the pipes, being able to expand outwards as well as widthways?

Ahh, filler faom my favourite diy stuff!
It should do the job nicely.
Follow the instructions on the can.
Even if it expands too much, mine always does, it can easily be sawn off
when set with a flat woodsaw or large hacksaw blade.
If you want the repair to lokk neater it can then be skimmed with polyfilla
and it takes paint.