Platicodon Grandiflorus or balloon flower, very pretty.

They come in blue white and pink

yes it is Platycodon sometimes it comes through the winter well other times it doesn't. so if you want to keep it then keep it frost free over the winter.

Thanks, everyone! I must have planted this in the pot last year, but I don’t remember it at all! (Ah well ...) When it first re-appeared I assumed it was a weed, but it grew right in the middle of the pot, seemed very well-shaped so I left it (well, I probably would have done anyway). As it grew I realised it must be something I’d planted last year. I am very pleased to have a name for it. Thanks again!