Cheers fizzbuzz, it's what I did in my last job but I don't know everything by a long. looong way! Can't find the thread you mentioned?
Vava, what traders do as goodwill gestures for customers and what the law says they are obliged to do are two different things. Did you buy online? If so, the law says you have 7 days following delivery to change your mind and send goods back (unused) but you have to bear the postage costs.
If the item is faulty, misdescribed or not fit for the purpose then you have the right to send the goods back for a full refund or replacement and be reimbursed any p+p. The seven day limit doesn't apply but you would have to notify them within a reasonable time (not much more than a couple of weeks although the law doesn't specify).
In your case, because you didn't notice it was the wrong item (it is your responsibility to check) and the item has been treated in a way that would suggest you have accepted it i.e used and washed. This affects what you are entitled to.
Now you can ask/demand whatever you want but whether you get it is a different matter. You can even threaten them with bad publicity. Most traders will do more than they have to just to keep customer happy but some won't and, in your case, they are entitled to refuse a full refund or a brand new replacement - miserable beggars of course but there you go.
All I can say is the law gives you certain rights but it's another matter to get what you want if the trader decides to be a complete git.
Your options are:
Send a recorded delivery letter as advised earlier. Don't forget to tell them what you want them to do after you've explained the situation. If no response take legal action (Consumer Direct can advise you how to do this and you can get the documents from the local county court. But bear in mind you might not win and the whole thing could cost more than the grobag and is a lot of hassle)
Threaten bad publicity if you must as it sometimes works.
Put it down to experience, don't buy from the trader again and sell the item on to recoup some of the money.