I wedged the rear section in place with bits of folded card, then sprayed the roof and headlining together.
This seemed to work pretty well as I avoided having to manoeuvre the treated headlining through the door and into the back of the car.
For the front my wife helped with getting the glued headlining into the car and aligned.
You can see the foam roller I used to smooth the sheets into place, working from the middle outwards as recommended.
I used the whole can of Trimfix, so hopefully that's enough to prevent it dropping any time soon !
Thanks to Tony for the advice and encouragement to finally take the plunge with this job - just the panels to do now
Cheers,