Thanks chaps.
The reason for this post was - the top nearside of the rear screen seems not to follow the contour of the aperture as well as it might.
I wondered if the rubber was getting a bit tired, hence the not-perfect fit at the top nearside. It may be that the screen was not a perfect fit in the first place, but the rubber was keeping it in line and now the mis-alignment is showing itself as the rubber yields under years of holding it correctly - maybe?
Does anyone know the thickness of the plexi screen and the thickness of the alloy lip of the aperture? - can't order any new rubber until I know and I don't want to take the screen and rubber out yet.
Does cut-to-length mounting rubber behave itself around the corners when bent, or does the mounting rubber have to be pre-formed, as it were, if so, is it available anywhere??
All help gratefully received
Glenn