I found a single packet in my search of a different M&S today. Looked over my shoulder to make sure I wasn’t being watched then hid it under the rest of the groceries in the trolley.
Found a helpful assistant who was stocking shelves. Turned on the charm and asked her if there were any more packets in the stock room. With a wave of her portable scanner she told me I had the last packet available. She took pity on me and told me when the next pallet of them was coming in. You don’t think I am going to reveal that secret here, do you?