Following Al's advice, I removed the scuttle and screen bottom trim from the Opel ('scrap shell'). Autoglass replaced the screen on this car last year, and I watched the man do it. I took no photos, but helped where invited, had the scuttle off when he arrived and put it back afterwards, sealing it thoroughly as OOF instruction.
The bottom trim is one piece rubber, and is attached to the screen with black adhesive; it's attached to the body too, with the same adhesive. Possibly he was generous with the adhesive and it got everywhere, but I see no harm in that.
Annoying thing is that I had 2 men from Autoglass here last Saturday to make good a screen replacement they had done on Jonny's 2.2 in May, the upper trim having come loose and flapping around. This car is now here awaiting head gasket replacement. I had the scuttle off the car with the loose screen bottom seal when they arrived, and asked them to glue it back in place. They said it was a push on.
They also said the upper seal was a push in job too, pushed it back in, agreed it looked insecure, so produced a giant tube of black adhesive and glued it in.