Thanks to all you list members who replied to my request for information on replacing the screen in the pop top on our Westy. (Lots of experience out there on the List).
The consensus is clearly to hand sew and/or glue in new screening material, using the existing screen and overlaying new screening material. Lots of cautions about making sure that the lower seam is waterproofed.

Tom Hewitt
82 Westy
67 Beetle