The Imerys Clay County Marathon & Half Marathon are on the 19th May, St. Austell, Cornwall. They were new races introduced last year I think. I've signed up for the Half. I didn't take part last year, but they are quite tough I believe and take you through the china clay works, so it'll be like running through a lunar landscape with some spectacular views. Both courses are a single loop - check out St. Austell Running Club website.