Popped into the M&S foodhall to grab some shopping and paid at the self checkout. Pressed the "Using my own bag" button on the screen to get started and then had to tell it I was using my own bag on the next screen. Scanned my stuff and pressed the done button so I could pay and got asked if I was using my own bag. ***, who designs this crap.
So... were you using your own bag? Not clear from the story.