Thanks Gary for that seed of doubt! It's true that I don't remember planting any Rudbeckia, so I'm definitely open to opinions from those more experienced than I!
Yes, the seedlings are self-sown, and are definitely grouped over about a 40cm area, but are nowhere else in the garden. So it would seem likely to be a self-sown flower rather than a weed, but I have so many weeds in my garden that I wouldn't put it past a cunning weed to try and sneakily pretend to be a flower!!
In answer to the 'old dead wood' comment -I don't remember there being anything dead looking there, but I have just blizted the garden in a frenzy of weeding and might have pulled it up. I wouldn't have pulled anything vaguely flower like up though, and don't think that I pulled up anything more substantial than lots of stinging nettles, couch grass and other small weeds.....
Thanks again for all your comments. It's very helpful,
Rachel
