John Rumm wrote:

How about casting it in concrete? Build a former with shuttering ply and
then fill. Once set insulate the inside and insert second former and
fill it again so the insulation ends up cast inside.

Hmmm... I hadn't considered that. But it does sound like a lot of work!
Given the size of it, I'd want to get readymix, and because it has to be
done in two shots I'd need two partial loads.

Just been looking at the U values for thermalite - for 9" thickness it's
0.08, which doesn't compare favourably with 50mm of kingspan (0.022). If
I do it in block, I think I'll have to figure out a way of insulating as
well.

--
Grunff