I'd say it is some kind of dianthus judging from the foliage, but as soon as the flowers open you can be sure.
(You can't put a direct link in a question but people can copy and paste the web address into the browser bar. You can insert a picture though by up-loading it when asking a question.)

Dianthus? Really? That tall?
The flowers have been green for a longish time.
The picture is big so that people can have a proper look and is too big to post.
Thank you for the answer. I will do some research.

Looks like a michaelmas daisy to me, but we won't really know til it flowers.

You can upload pics from your computer to the question when asking it - or add them later.

So I should leave it for now? How could it have got there? Bird seed?
The picture is huge, but, if I'd thought, I could have posted a thumbnail.
The lower, older leaves are sort of arrow shaped, but that doesn't show very well in the picture - like ivy, but longer and narrower.

It is indeed a type of Aster. The small bracts around the capitulum (flower head) are very indicitive of this genus. The branching stems also look like it. I think it is covered in downy mildew too from the look of the picture (another sign that its an Aster Lol).

Thanks all. I will leave it and see what appears.