Have got a couple of quotes for coaches and the cheapest works out as follows:
Coach departs Friday 20th July at 11am from Baker St.
Coach departs Monday 23rd July at 11am from Oxford.
At a cost of:- £15 / person return only! (can't do single journeys - too confusing)
I'd definitely need the money in advance this time coz it wiped me out for a couple of weeks last year, and I have other festivals to pay for!
If you are interested, say so down there VVV and I'll book it based on uptake.