OK, progress update - takes longer to upload pics to Photobucket than it does to do the jobs
Started with gluing some battens to the roof, want to make the lining follow the side to side curve of the roof but keep it pretty flat front to back. Used Sikaflex EBT+ available from Screwfix although a bit pricey there - seems strong just takes a bit longer to cure than i would like. Also put some flashband between battens to dampen the roof noise and will put in some foil insulation before final fit
Did this on and off over a couple of days whilst also sorting out new brake callipers and getting it ready for the MoT (which it passed

)
Bought a 8x4 sheet of 6mm exterior ply, if you are curving the ply with the roof then 8x4 works out about 1/2" short each side, but that is something i can live with - will work out a way to cover the small gap later depending on how it looks. i would rather this than have to make the roof lining out of two seperate panels.
Plan was to start the roof lining from the rear and work forwards, so first off used some of the old lining to make a template for the rear, then clamp it too the new lining and cut.
Got it snugged in nice and tight at the back.
I am having to do this mainly on my own so having to make up some supports to hold it in place whilst I move and measure. At the back it is supported on a workhorse and blocks but at the front i deflated an old exercise ball and am using it as an air bag/jack

Its great as it rolls so makes sliding the sheet in and out so much easier and when I want to fit it up to the roof I just inflate again!
With it in correctly at the back I then needed to cutaway around the sides round the middle cross brace. made a template, transferred to wood.
Could have been a bit closer tbh but will do something to cover the gaps.
Then inflated the "air bag" so i could see what it looked like fitted.....
Begining to look decent, now need to tidy up the cutouts a bit then can have a look at the front.
Cheers,
MM