Borlotti, there is a seed swap / potato fair every year in London, but I think it was in Jan (I can't remember where it is exactly, it's either in S London so you'd have to get your passport out, or it's in E London so you'd need to take Kev with you!), so you'll have to put a date in your next years diary!
Robert, keep trying. I'm starting one on our allotments, and to be honest if I get 6 people I'll be chuffed, I reckon you have to start somewhere. When I did one for a few friends last year, I realised I hadn't given it a great deal of thought, so having joined in the seed saving circle on here and going to a swap I'm a bit more clued up! One idea I did have was to tack a seed swap onto another event, a plant sale or a talk or a seed saving talk / swap. That way you might get some more people along.
1066 :)