Plenty of plastic bags to wrap your wet stuff in... and it is wet on the site... I'm on the opposite side of the river from the site in Caversham and its a quagmire.
1. big golf umbrella - can get one for about �12
2. plastic plate and cup
3. wet wipes AND baby wipes
4. water bottle
5. torch
6. wellies
7. dry shampoo if you've got enough hair to warrant it
8. rug with waterproof membrane
9. NUROFEN
And try to avoid bringing clothes which are 100% cotton, they take ages to dry.