I was at Morrisons in Kings Lynn (Near the train station) the other day. I spotted these for £3 each. They normal sell for £5 and sometimes £10.
There are next to the other counter in the middle of the store.
Worked out good for me because Duracell sent me a £10 voucher :)
Please Note : I am sorry if this has been posted before. I can't say for 100% they have any left but I only went on tuesday.