polycarbonate seal strip?

My polycarbanate roof on sun room/conservatory is leaking. The glazing is laid on foam of the rebated glazing bar. On the outside is a rubber (brittle and perished) 12mm flat seal that is slid under the glazing bar with raised lip that is pressed against the glazing bar to form the seal. Where can I get in the U.K. both seal and foam? tried local ( West Sussex )with no luck.

It's a crap design but can't afford new one.