How much give is there in PIR foam when pushed between battens? That
is to say, if I use 47x38mm treated battens do I use 40mm or 45mm PIR
foam?

The solid double brick wall is "distinctly variable" re bellied wall,
bricks tilted, however obviously the battens will sit on top of the
wall.

I assume 40mm, just wondered what people have found from experience.