Thanks so much for your reply.
It is a stud wall.
It is a stud wall between a bedroom and the staircase, which goes up the middle of the house.
we've done quite a bit of sanding elsewhere in the room on other walls and haven't found these fibres to our knowledge.
They seem to be quite...
Here are the pics - now I've had another look, it is white/transluscent fibres, which seem to be embedded in the surface as well as loosened into a big lump by the sanding, which looks a little like cotton wool.
This is on an internal wall.
any help and thoughts greatly appreciated.
We've just moved and have been preparing walls for painting, including filling holes and sanding down lumps.
One of these lumps revealed some fluffy fibre when sanded, which appears to be midway into the lump.
So all work has been haulted until we can identify what it is.
We have artex on...