Just investigated this.
One-day tickets are 50 quid (which you can pay by Paypal from the site).
Trains run from London Waterloo to Salisbury and take about an hour and a half, with a bus ride that takes you from Salisbury to the festival (about 17+ miles away).
Gatwick to Salisbury is about a 40 minute to an hour train ride, changing at Clapham Junction. Heathrow Airport is about 45 minutes (or so) via the Picadilly Line (could be hot in the middle of August)!
Obviously, a hotel will need to be booked at Salisbury for the Sunday night, unless anyone's driving back?