Thanks Andrew.
I knew there was a piece of cord/string involved somehow. However, it's probably not wise to "learn the ropes" on a windscreen which could be very hard to replace.
We've had some discussions about V8 Bristol windscreens before on the BEEF mail list, and probably before that on the the BOC forum, and unfortunately I can't recall anyone ever identifying the front or rear screens as coming from another vehicle.
The but joint at the bottom on my rear windscreen isn't very well butted together and I'm concerned it will let water in (not sure where it would go). I think this may receive a dollop of windscreen sealant for the time being!
Also, with the front screen, the metal surround which fits into the outside of the rubber moulding is springing out at the top joint - the centre joint piece was held in place with silicone rubber. Apparently there is a brass frame under there which goes all the way around the screen and it has been suggested that this has been bent when the screen was removed at some stage and it may be able to bent back into shape without removing the screen.
Can anyone shed any light on this. A cross sectional diagram would be really nice