Stuart42mac - Well, how did you get on then?
Tricialitt - For some of the ultras, the cutoff times are more a general guide than something set in stone. For example the Glasgow-Edinburgh one had a nominal cutoff time of 12 hours, but if you look at the results on their website, they kept the finish open long past that and the last man in took about 13 and a half hours. Not saying all ultras are like that, just that some of them are. If you're behind the cutoff times and look to be struggling, they'd probably pull you out for your own safety. If you're looking good but just happen to be rather slow, and the weather conditions etc are fine, chances are they'll let you continue. If you're interested in a particular event, compare their stated cutoff times with their previous years results to see if they allow any leeway.
P.S. The D33 (Aberdeen) doesn't have any cutoff times, just says it's for runners not walkers so go as fast as you can. I got to the marathon point in 4.40 and finished in just under 6 hours. The last finisher in was about the 7 and a half hour mark and loads of the faster runners had hung about to cheer the last folk in which was really nice to see. I'd totally recommend that one as a first ultra - everything about it was excellen!
Edited: 14/04/2012 at 11:53