Hello Ian,
from your posting I cannot work out if you are doing the craft fair as an individual or a club. If your club was affiliated or associated with the AWGB and they got insurance from the Association and if the craft fair attendance had been minuted at a club committee meeting previously the chances are you are covered. If not you are not covered and as you would be classed as a trader then PL insurance is a must, without it if someone made a claim against you your house could be sold to cover the costs.(assuming you don't have £5million in the bank (apologies if you do))
I would advise taking out PL insurance, as Pete said, Ian Wallace is the best person to contact.
regards
john BHT