have you tried ebay? there's show jackets on there. i got a very lightweight navy showjacket with pinstripes a few years ago, and it was pretty cheap, i can go and check what make it is if you like? i'd freeze to death wearing it in winter though, it's really summerweight.
what size are you? I have one to sell; it's Caldene, tweed but grey in tone with navy velvet collar. Will fit size 8-10. PM me if interested, I was going to take some photos of it tonight anyway as it's probably going on ebay!
Don't really know Royston I'm afraid, have you tried an internet search?
I will try and take some pictures of my jacket tonight, need to anyway for ebay so just PM me if you want me to send you them.