Belated thanks to both Owain and Steve Firth.
I only just noticed your replies when I downloaded a few more hundred headers - I had not noticed anyone had come back to me on this until now, so cheers are long overdue.
Steve, I might have asked about silicone rubber sheet if I had thought there was any chance of getting it, and that it had enough tear resistance - it might indeed prove to be superior to PVC and may be better at laying flat and staying put ( I too have env reservations about pvc - though I might be somewhat allergic to the rubber's plasticisers/acetic acid too...); and Owain, the Allplas outlet is not far from me at Letchworth, and does seem to do the clear pvc version, so I should be able to go and have a look and see if it might be suitable.
Thanks very much for finding the links for me (I did try: honest!).
Cheers,
S