Forgot - Under the Sale of Goods Act - you do have the right to inspect the item, so if it is incorrect or damaged when you inspect it and you are unable re-package it.
Little tip for thse store that say 'proof of purchase required' and you have lost your receipt. This does not have to mean receipt unless they specifically say this . Walk up to the counter and say to the shop assistant "hi, do you remember me...I bought this......last.......". When they say yes because they do not want to be rude, they have given you your 'proof of purchase'.
If they try and pull the wool, just explain what proof of purchase is as I have.
Regards,
Steve