Is there a time limit in return policies in the English law?

Hello everybody,

I bought a DVD Box at the zavvi online store. Date was in the beginning of May 2011. It is a Box containing five seasons of the TV-show "The WIRE". On every DVD of the fifth season the first episode is missing. The fifth season has four DVDs and 10 episodes meaning that 4 of 10 episodes are missing.
I contacted zavvi but they told me, I can't return the Box or the fifth season due to being out of a 28 day return policy. They write something about that in their terms of policy, but I am not sure that this is legal. The Box contains of about 3500 minutes of film material. I doubt that I have to check every single DVD in less than 28 days.
Unfortunately I am not living in the UK but in Austria and have no idea about the english (contract) law.
Can anyone help me with this?