Cresser, did this look completely different when it was little? Like this, for example?
The above is the picture I posted on here to find out what that little plant would grow in to, - a whacking great tall weed called a fat hen - the ones you see in fields with the red bobbles on great pointy stems To me, your plant looks like one of those before the bobbles (which are all seeds, and there are up to 20,000 of them per plant, apparently!) form. I hope I've downloaded a link which would show you a big one, which I think looks like your plant. There are lots of dire warnings on the net about how aggressive they are and the ones that I now have came from expensive compost.