Go in again and write down the names of everyone you speak to. Make sure you escalate your complaint to the floor manager, then the shop manager.
If you fail to get anywhere ask for the head office address (it has to be displayed in shop anyway, but still...). If you still don't get anywhere write off a stern letter to their head office with all the notes you made, and give them 14days to give you a refund (on your way to the post office report them to trading standards). After the 14days take them to the small claims court.
Usually once the store manager sees you're making notes about everything they'll give in because they know they don't have a leg to stand on, and they'll get a rollocking from head office and trading standards.
They might say that you bought the wrong goods, i.e., you bought a size small T'shirt instead of a Medium, in which case you've got no case and you're not entitled to a refund (unless there is a fault).
Here though the shoes should've gone straight to the till scanned and into a bag without you seeing the size, hence you were mis-sold. Make sure you point this out.