Send the tent back as it is most company policies refunds cannot be given without the return of the faulty good, however get the delivery driver to check and sign a document with the damage written down, that way you have proof that someone else has witnessed it.
CEX are quite good with stuff like this is my experience, could they not give you the difference in credits to use at the store to but something else??
I would just send the items back and get a refund as you may be wasting your time trying to get them off play.com for the same price?
Have you tired ebay or some of the other websites.
I had the same issue last year, After looking forward to getting my item the seller did not send it out and after waiting 2 weeks I contact ebay who refused the money and the seller says he sold it off ebay. Only thing I could do was leave negative feedback.
This happens to me when you make "irregular transactions" Call the 0845 number on back of your card and they will take the block off your card after you tell them it was you using the card.