Incorrect prices for items supposedly on sale

I purchased 2 items from MrP Sport on Monday, i was charged full price despite the items being on sale. The one item i ordered on-line and returned for a smaller size but surely it should have been picked up on the system. I had to already point it out to the staff that the prices weren't the same for the same item. The staff was very relaxed looked more tired and concerned about going home as if the store was overflowing with customers! Which it wasn't. Also they had a staff members cool drink in the fridge, a can of SCORE, which another customer wanted to buy was told it wasn't for sale. surely this then shouldn't be in the fridge. I would like t know how do i now get my refund on the difference i paid for the items? Surely its not fair that i paid full price for items on sale! and no i cant drive back to the store from Mitchell's plain to canal walk.