hi thanks for your reply, unfortunately I don't really know anyone who
can help me. I have ended up drilling one of the holes further to try
and see what the wall is made of; the drill got about 7cm in (can't be
plaster and lath can it?). The top layer which is about 2.5cm thick
consists is slightly yellowy greyish with grit and I think I saw hair
in it(?). After that the wall seems to be more greyish but is still
just coming away with my hand as dust.

Can anyone tell me what this is and what I can do? or should I just use
lots of filler and forget it?

Thx Rich