This has been growing on its own in the garden lately (London), and intriguing me as I haven't noticed one before. About 3 ft tall now, with smooth round stem except for the flowering part which is rough and grooved.
Thanks Steve, seems like a good match - I'll have another look in the morning. I suspected too that bird seed might have something to do with it. It'll be interesting to see if the birds take a liking to it.