Just got around to doing some insulation calcs.

This is all for walls, just using idealistic values.

Thickness Polys Polyu
50mm 754W 452W
75mm 503W 301W
100mm 377W 226W

I suspect 100mm will be too much in terms of costs/space loss.
I'm currently thinking 3x2 tanalised timber mounted vertically between battons with ties onto the wall in the middle for support (the walls aren't straight enough to mount straight onto!), have an air gap between the brick wall and 50mm insulation, then face it all of with OSB3.


Ebay cost of enough 50mm rigid insulation panels to do all the walls is £384, which isn't too bad.
And I could always add 25mm polystyrene to space them from the wall, for not much additional cost.