Try a different store, or contact their customer services department directly. Make them aware that you've come across pig-shit staff in the original store and thought you'd try and find someone more helpful.

If you've opened the box, that's hardly a problem as that's what most of their 'b-grade' stock is - stuff that's been returned or not complete. If it's not fit for the purpose for which it was intended, or sold to you under false pretenses, that's when you can go down the legal route through the Trades and Descriptions Act.

Your other option is ebay your card and try to make some of your £60 back.

Can you slander a company? Yes you can. Don't expect them to be your biggest fan though. Can you publish libel? Yes you can, but expect to be taken to court for it.