The material used was PolyPly supplied by Plymaster but phone ahead, they don't always have stock but they are happy to order it if you phone a few days ahead.
The bits that required a slightly different finishing approach were at the aft end where the headliner meets the wood, and around the forward hatch. Both were finished with white plastic angle, the latter sealed with sealant to stop any water from getting above the headliner.
The replacement headliner was a lot thinner than the original so I needed to put spacers in a few places so that there were not gaps where it met existing timbers and trims.
This turned out to be a time-consuming but satisfactory job. It really does look good now.