I don’t know what happened to my bike yesterday but, when I unlocked it in the evening, I found the chain had somehow jumped out of the derailleur cage. My guess is that it got tangled with another bike and pulled hard when the owner came to cycle away. However, the gap was too hard for me to get the chain back in, at least by the side of the road in the dark.
I discovered that it was possible to cycle even with that part running over the metal of the cage rather than the jockey wheel inside so, with a rehearsal to get to and no way to phone Jane to let her know I’d be in late, I cycled home although more gently than normal and largely avoid gear changes.
This morning, it was a fairly easy fix with the assistance of some pliers to bend the cage a little to readmit the chain. It seems to have survived unscathed (and the bike probably benefited from the extra cleaning and lubrication it got this morning). Strange, though.