Hi Ginglygangly, thank you for the feedback. I've got a lot of flowering tobacco seedlings, that I'm growing so I don't think it's that. The ones I'm sowing are all taking a long time grow, whilst this thing growing not far away has shot up.
Here's another picture, it's about five to six inches wide leaf tip to leaf tip.
Here it is next to my flowering tobacco which came out well before this, so I hope it's not the same or I'm doing something seriously wrong with my seedlings.
I know my seedlings are overcrowded I read that you should pinch out the ones that aren't as strong to soil level. I'm just waiting until I dare to do that and prick them out and repot them.