didn't think the registration thing would be practical for fell races, but scanning a barcode attached to a race number at the start and end might be (but wouldn't account for people going missing mid-race as it wouldn't be apparent until the race had "finished" and you still wouldn't be able to account for people who get lost and find their own way back "home")...at least you'd know the exact number of people starting.....
5k is a bit short for me!
