Pennards since 2007
The Somerset Cider Bus (it really is a blue double decker bus which serves cider) is round the corner from the Pyramid Stage (see map I7 PNG, 1307.53 KB or google maps).
It serves gorgeous Burrow Hill cider, as well as Hot and Spicy (a mulled cider, perfect for the Christmas Eve Celebrations) and cider brandies.
Don't worry, if you're not a cider drinker, there's a large beer tent next to it.