Hopefully I can now shed some light on the Mk1 rear quarter windows.
Firstly, Harvey has told me that I am right and that it is the VW Sirocco glass that was used. However he did not state which mark it was from.
Secondly. They paid a cut glass crystal maker to round off the rear edge so that it would fit the Vauxhall Chevette rubber seals. These seals were from the 2 door version of the Chevette.