Another pretty weed .....

..... but what is it?  There's lots of it at the bottom of the garden, grows about 1 metre tall.  I don't remember noticing it before, but then, it's not very noticeable in small quantities because it's quite leggy with thin, smooth stems.  I'm sure it can't be rare, but I have been unable to identify it using my old fashioned book, or using the WWW.

I've put extras of the flower and the leaves in case anyone should care to take up the challenge of identifying it for me.