Well there have been developments. Turns out that I didn't pay at all. Today my till friend said that I had attempted contactless payment but, unnoticed by me, it hadn't worked. I had popped my headphones in and couldn't hear her as she shouted after me. A security guard offered to go after me but she said "leave him, he's one of my regulars".
So fuck you the people who said I paid 30p for my shit bants. Turns out I was paid £4.something for it.