"Last year, there was a v. limited resale in early May following the last date for ticket refunds. Don't yet know if there will be this year."
From the official twitter page -
http://twitter.com/GlastoFest/status/59564170819010560
I guess it will depend on how many people do ask for a refund, if it is very few it will be too much hassle for them to run a resale for only a few dozen tickets.