Correct me if I'm wrong, but aren't there certain riding restrictions when it comes to Snowdon?
I think they limit the times you're actually available to go and ride, so that you don't run over walkers. E.g. before 10:00 and presumably after a certain point in the afternoon too.
EDIT: It's actually a voluntary ban between the hours of 10:00 and 17:00, between May 1st and September 30th.
http://www.singletrackworld.com/article.php?sid=987