Thanks for the suggestion daveyboy1967. Good idea.
in the end I tried leaving the car with the rear facing the sun so that the plastic of the replacement window would get really hot.
That, along with some repairs to the zip, which had detached from the hood in one area (I used a Speedy Stitcher to stitch it back into place) seems to have done it!
There are some creases, and I did have to use some clear sealant to improve the rubbish gasket that came with the window, but it’s ok.
Certainly worth a punt rather than shelling out for a new hood or travelling to Coventry to get a new window stitched in.
At least now it’s waterproof and presentable.