There is no fixed seating so it isn't like Olympia or HOYS. You can get tickets on the day - although they can be a couple of quid more. There are usually loads of 2 for 1 type offers in the local press before the show.
Put it in your diary so you don't forget to go - but I'd suggest leaveing ticket buying until closer the time.
I expect I shall be collecting the free tickets in the local media (nearer the time) for readers on here - like I did last year. See my previous post(s) last summer - Hickstead tickets.