Osprey Renn at Gooutdoors. £99.
65L perfect for Glastonbury and inter-railing.
Top and bottom loading, good support, rain cover, snack pockets etc but it is lacking a front pocket if that's important to you?
Check weather on the day but shorts will be fine. Plus it depends on the temperatures you are used to! Last time I brought a denim jacket with me all three days and tbh I didn't need it, but it was useful to sit on. I wore it most of the time though because I didn't want to carry it around. It was about 16c at night if I remember. Personally I would not bring a hoodie, too bulky, but if you are comfortable wearing it then bring it.
It does cool off at night (just checking weather reports and they're saying around 8° cooler). I bring a thin layer.
Amazon stage to main stage, depends how busy it is but around 10 mins. I reckon there'll be a fair few going Deftones to Pulp...