I was planning to dig it up as I quite like it and it's in the veg bed so can't stay there. No sign of it flowering or seeding and it's been there all summer so would think if it were an annual it would've gotten to that by now.
I shall see about digging it out tomorrow. I'd been putting it off as the nigella will throw a big tantrum and give up once disturbed. Shall get a better photo when I do dig it out.
It is indeed fanning from a central sort of point, as blairs said, so hopefully it isn't bamboo. It is quite near to the pond (maybe six feet) so your water reed suggestion was interesting, maybe carried on a frog?! There are a lot of birds bringing all sorts into the garden as well, but I've never seen anything like this.