The message
from (N. Thornton) contains these words:

B. Apply expanded clay in lime plaster to the interior. This gives
insulation with no vapour barrier, retaining wall breathability, and
without the damage cement sometimes causes.

Not a solution I have previously heard of. How effective is that plaster
mix as insulation?

I am in a similar position to the op other than I have 2' thick rubble
filled stone walls and no external render.

--
Roger