Expired good on rack and will not let me return

I recently bought a caremello slab and a strip of simba chips from our local spar and when I was at home I opened them and the chocolate was white crumble and the chips tasted strange so I checked the expiry date and the chocolate expired in 2014 and the chips a month before and when I asked if I could bring the good back to exchange them they said they cannot help me because i do not have a sllp, who is going to keep a slip for a chocolate and chips, I am very sad as we all work hard to earn our money and then we get sold goods like that and cannot even take it back