Thanks for the advice so far. A little more background:

* The shed is already installed. It backs up against a rough stone
wall i.e. I can't get behind it.

* The internal wall battens are about 44mm thick, the roof battens
about 66mm. I won't build out thicker than that or the shed will
become too claustrophobic. What is the best insulation I can provide
in those depths? I realise the shed will need heating in the depths of
winter.

BTW the Frame Foil expert at Sheffield Insulation said it would be
fine to put plasterboard directly on top of the foil i.e. no air gap
on the internal side. Would you disagree?