Thank you for those tips, I'm glad it's not just me. It came off the largest cog on the rear deraillieur and got wedged under that pie plate thing at the back, so even with as much slack as I could get it wasn't coming unstuck. Wheeling it home did seem to help though, I got it sorted in the back garden.

Weather is sunny but cold tomorrow, so I'm going out in my new tights! I'm starting to worry that I'll forget how to ride if I leave it another week.