Grunff wrote:
JK wrote:

Just looked into it - a layer of Celotex insulation is 45mm, they
specify a 25mm gap, then plasterboar at 12.5mm so you lose 82.5mm
from the size of the room, which was not big in the first place.
Any idea if there's a more compact solution?

Foam backed plasterboard. Never used it, but it is widely available.

Found it. Kingspam Thermawall, very cheap, variety of thicknesses. Cheers.
Still scary though, all that stuff about window and door reveals, no idea
what the terminology means!