If it's really sunny you'll need a hat so how about this 'Ditsy' trilby from Accesorize (was £20, now £10)? The beauty of a hat like this is that it'll squash into your bag and won't look the worse for it.
With trenchcoats being the exception, rainwear is always a stretch when it comes to fashion but it is a must for festival-goers. Given that the endlessly chic trench can be a bit more bulky you'd be better off in a Lightweight Parka like this one from New Look was £34.99 now £24.99. It is equally perfect for screwing up into a ball when the sun is inevitably shining.
Avoid the awkward tan lines by wearing a strapless dress like this soft denim beauty from Sandro (was €175 now €87).
And finally footwear. Whilst it's paramount that your feet are comfy, your shoes will no doubt get pretty trashed so if your dry weather shoes aren't a washable Converse type affair (you can get Light Oxfords at Schuh for £26.99 rather than the usual 34.99), stay cheap and go for something like French Connection's Nadalia trainers (was £35 now £25).
Last of all don't forget knee-high socks to avoid welly-rub and tights or leggings as back-up for when the temperatures drop the evenings and you can't be bothered to trudge back to the tent. Finally add rip-off Rayban Wayfarers (avoid the heartbreak of sitting on the real ones by leaving them at home) pile on the charity shop bracelets and get into the arena.