I'm also a member of the National Market Traders Fed ~ as well as the public liability and product liability cover it also includes employers liability, which is useful if you ever take someone along to help you out. I've also got cheap car insurance through the NMTF which includes me travelling to markets as a trader (and many won't entertain this occupation) + I got my merchant account set up for free by telling the bank I was a NMTF member.

I know a few people who do craft fairs and get their public liability insurance through Ian Wallace, who insure crafts people.