Refund for substandard product

Yesterday (4 October 2017) I bought a hake from Fishermans Deli. When I got home I detected a strong smell from the package, but I nevertheless proceded to cook it. My daughter tasted the cooked fish and immediately said it is off. I found the brown paper in which the fish was wrapped with the price sticker on (date, price, etc). So I took it to the shop in Dunkeld West and asked for a refund. After much toing and froing, I was told that their "new rules" determine that they can only refund me on presentation of the payment slip, which I did not have. There was absolutely no reason why the price label could not serve as proof of purchase. Fishermans Deli did not deny that the fish was off (and judged by the smell, it could not have been the only one) but they obstructed me to get a refund