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.

--
Grunff