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?

Both holes are in the cupboard which now surrounds the combi. Out of sight,
but hate the thought of giant spiders making their home.........

TIA

ZD