Very simple:
I purely made this for my own convenience. I got the data from http://clashfinder.com/s/electricpicnic2012/ and I take no responsibility if any of it is wrong and you've missed your favourite band.
You can use the app to favourite bands and if two or more clash, they'll be greyed out.
It's ugly but you never know, maybe Electric Picnic will pay someone to create something professional for next year :)