I have just returned from shopping in the Locks Heath Co-Op to find TWO items I bought have passed their Use-By date. I will be returning them of course. Has anyone else experienced this?

To my knowledge, no. They are normally quite good at keeping on top of these things. Did you get the items from the discounted section? The area has usually got goods that are very close to their sell by dates and are sold at huge discount prices.
Don't forget to complain and get the fuel and time and trouble money that they owe you.