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?

The trouble is that it will expand forwards out of the hole as well, so
you'll either have to press it back while it's still soft, or cut it off
when it's set. I reckon it would be less messy, and cheaper, to use plaster
or even Polyfilla. You shoul;d be able to do it with a small knife or
somesuch even with the pipes in the way.

Rob Graham