Try http://www.ctt.org.uk click on events,filter by distance, 24 hour, Nat 24hr, Mersey Road Club 24.7.10 is the only event for this year. I did the National 12 hour last year, it was the hardest thing I've ever done, same event that Wilco did 302miles, I managed 225.52. Very good atmosphere and constant encouragement from marshals and other competitors support, you will love it :D
Edit;damn slow typing!!!!!!
p.s they never get a full field so as long as you enter in time, you'll be fine.