The message
from "Mike" 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 use it for floors as it needs to be a lot thicker than Kingspan/Celotex or
even Rockwool. LECA (Optiroc) in a lime/sand mix to a foot or so depth
definitely does lower heat loss through the floor though. A friend did put
a thin layer of it as an outside render but this was on a 'best attempt'
basis rather than trying to meet part L.

I had a google but couldn't find any reference to its use as wall
insulation. One of my round tuits near the top of the pile is to dig up
and re lay my solid ground floor but I think I will avoid the expanded
clay route as I would probably have to dig out to below my shallow
foundations. I can't afford to raise the floor level at all as headroom
is aleady minimal.

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

Same here.

I note you have gone the batten out route but I have small rooms and
particularly in the kitchen, smallest bedroom and bathroom no space for
more than the thinest of insulation. One of my earlier jobs (still not
plastered out because of the insulation dilemma) was to increase the
size of the bedroom at the expense of the bathroom so space is really
tight in there now.

--
Roger