If the stems are three sided then it is a Sedge. From that distance it is hard to really make an id, but it could well be Carex pendula which is a terrible seed weed.

Luckily its definitely not Carex pendula, that has grass like leaves, this plant doesn't. No idea what it actually is though, might be as a result of a dropped bird seed

My thought too Bamboo, dropped bird seed!

I'd remove it, or at least those seed heads before they fully mature and start dropping, otherwise you'll be weeding out hundreds of them next year...

Definitely from bird seed as my neighbour has it growing along with foxtail millet & brown top millet, from his bird table, so I guess it's yet another type of millet. I'd like to know which one.

Yes, it does look like a millet.

I think it is a Millet too.

Right, so it seems to be a millet from bird seed. We do have a bird feeder and heaven knows what seeds are in the mix we use, so it may well have come from that, though the feeder is some way from the pot - but birds do fly of course!

I've taken Bamboo's advice and removed it completely. Much as we liked the delicate seed heads, one such plant at a time is enough!

In passing, in respect of Owdboggy's suggestion of a Sedge, the stems were actually round and hairy.

Thanks to everyone for the comments. We'll have to wait and see what decides to arrive next year.

I agree with Millet, it's probably Panicum miliaceum or Common Millet.

Thanks for the ID Land girl. I intend to grow one plant of the Fox tail Millet next year as I think it is really nice. Will have to make sure it stays one plant though!