Square stems and opposite leaves could imply Salvia. Did you have any of those?

Are those stems really square? If not, Maybe Ageratina altissima?

Maybe not. I was looking at the one top left of centre. But you're right. The rest look round.

Thanks - The stems are perhaps mildly ribbed, but I wouldn’t think of tjem as square at all. The piciture shows more than it feels.

Ok, well I think that confirms I’ve got work to do. Thanks!

They pull out easy, but be careful not to leave the roots in the ground if the stem breaks, or they will grow back. It's also extremely toxic if ingested. People have died from drinking milk from cows that ate it.