The latest issue of "Sight and Sound" has an article on this:
"Here's A Health to the Barley Mow:A Century of Folk Customs and Ancient Rural Games" a 2 disc DVD set from BFI out on 18/07/11. If you want to see where all the "pagan" dances etc in "The Wicker Man" came from then this would be a good starting point. It contains the excellent "Oss Oss Wee Oss" which is wild.
If you don't fancy 2 discs of Morris Dancing check out the magazine for the weird beard photos that make "The Wicker Man" look very conservative! Right I'm off for a quick wassail.