According to their own terms and conditions under section 7click here
"a)You have the right to cancel the contract at any time up to the end of 7 working days after you receive the goods (see below).
b)You do not have the right to cancel the contract if the order is for computer software which has been unsealed by you, or for consumable goods which, by their nature, cannot be returned, save where a fault is discovered which could not have been discovered otherwise than by unsealing the goods."
I read that as saying that if the manufacturers packing is intact you have the right to return them.
That said, if you do return it you will have to pay for the cost of the return and they are entitled to charge a reasonable restocking fee.
If the goods are as you ordered, i.e. you made the mistake by ordering the wrong item, then you don't have the automatic right to a refund.
On the site it says that they only offer credit notes for any and all returned goods. This is illegal, see click here "If the shop displays a sign stating they only give credit notes instead of refunds, they might be breaking the law and you could report them to Consumer Direct on 08454 04 05 06."
Unfortunately, this doesn't help you as the DTI also state:-
"Must I accept a credit note instead of a refund?
It depends on why you want to return the goods.
• If you have changed your mind, then the shop doesn't have to do anything."
On a personal note, I would not do business with any company which says that if the goods are faulty I can only have a credit note not a refund. Although this is illegal and can be challenged, it is easier and simpler to deal with companies who operate a proper returns policy for faulty goods.
All you can do is ask to speak to a manager, explain that the manufacturers seal is unbroken and that their T&C allow you to return the goods.