I finally have the insulation and a dry non windy day.  First job was to strip out the ceiling ply and remove the preformed plastic base that was in the over cab area.  I removed the preformed based as it restricts access to the space and reduced the amount of space available.

Base Removed

Ceiling panels removed and xtratherm stripped

5mm strips of wood glued to cross struts

Insulation sprayed on.

OverCab area insulated as well

You can see the 5mm shims showing which is what I will screw the panels back up to

Forgot one of the strips

Ceiling Panels reinstalled

In this last picture you can just see the join where two panels meet.  All nice and neat now.