One step forward, two steps back then three forward!
Got the windscreen down from the loft in the barn to check the fit (bought it around 10 years ago as the original was cracked). Unwrapped and offered it up to the opening. It didn't seem to fit, length is fine but it's around 40mm too shallow.
Bugger, possibly new screen required. However, on getting a supplier to measure one of their screens is appears to be correct so the rubber seal must be pretty wide (I have a new one somewhere so I will check later).
As I was in windscreen mode I decided to confirm that Lexan screens were illegal for road use. However, on checking there seems to me no mention of windscreen materials. So I popped round to my MOT station and checked. As far as they are concerned, as long as it is clear and not scratched then no problem. So, a few more kg shaved off (6kg). Result!
A couple of concerns:
Some race classes ban polycarbonate screens.
Glass screens add to the chassis/body stiffness.