Just turn up and enjoy it. You'll be in a charming part of England, at a perfect canal-side location, and you will hear some truly worldclass musicians. But do heed Arnie's advice. If you have a good place in the field you will not want to leave it to get more clothing. I usually took a sleeping-bag, which could be used as either a headrest or for warmth.

Oh, and if possible, go by barge!