I've been looking through the old forum and came across a posting by Robin Ellis about modifying a warning light from Halfords to use as the lens in the rear reflector. The result is quite realistic, although the lens has fresnel rings rather than the diamond reflector plates of the original.
This go me thinking and a quick look on eBay found me this version instead:
The part code is 0-609-25, and it's available on line from various car parts suppliers.
Adrian