Phil L wrote:
That's not stone, it's the backing plaster, probably sand and lime, and very
often horse hair, you may want to remove the lot and get it all plastered
properly

It did feel very sandy, know I know why: because it is!

If you've not touched any of the other walls yet, you may be as well not
bothering...this one wall needs re plastering completely - you may get away
with dry lining it.

I've only touched this wall. Dry lining sounds like a very good idea -
I'll look into that tomorrow, thanks!

Steve