A bit of advice please...

[robinbanks]

I also thought this, but maybe because we are using PIR for it to be effective there has to be an air gap?!

Not so in terms of insulation effectiveness. You're perhaps thinking of the very minor emissivity improvement by having clear space next to a silvered surface. If there isnt a gap, the silvering contributes nothing

If PIR needed an air gap, it wouldn't be used under screeded/concreted floors (which it is, preferentially to eps where extra compressive strength is needed)

It's not related to the material, but the shape; to reduce chance of the boards transferring moisture to the inner leaf full fill boards have a stepped edge that is intended to be installed such that any water getting on the board would have to climb the steps to reach the inner leaf