Is it always hot and sunny, then?

No, this is England. We are often lucky at Cropredy, but it's best to be prepared for a downpour. Waterproof boots and jacket should be packed, and you might want to consider a spare groundsheet so that you can sit on wet grass. A tip - leave a set of clothes in your car/van. If the weather is truly dreadful, you'll always have a few dry clothes to change into. Another point to consider is that even the hottest day can turn into a chilly night, so a thick jumper is worth bringing.