Looks like the RRP is now £19.99, but ASDA have it for £14.93, free Del and Quidco.
In the event ASDA sells out, you can also go to The Hut here: http://www.thehut.com/hut/9963866.product
for the same price, as The Hut and ASDA have the same prices for most entertainment goods, but numerous people have mentioned having problems getting Quidco to work with The Hut.
Edit: looks like a few people have been having problems getting it from Zavvi. Also, it's been mentioned that PC World will price-match this and give you 10% off the difference, effectively making it <£14!