Locust
There's an excellent website that should tell you all you need to know. It has a great database that allows you to look at all the markets in your area, giving details of costs, pitches, availability etc.
http://www.nmtf.co.uk/
If you are on a council run market you'll need insurance. Join the nmtf and it comes with insurance. It cost around £60 when i was trading. On most markets you'll be able to hire a stall for a day £3-4pounds.
Depending on what your selling, trading on a Saturday only can be difficult as youdon't get to 'turn your stock' quickly enough, and punters become bored lookingat the same stock every week.
Good luck.