Peter James wrote:
http//www.flickr.com/photos/phredde

They certainly are not dahlias, and I am intrigued and would like to
know what they are.
The local Garden Centre couldn't identify them. We think they came out
of a packet of Thomson and Morgan seed.

Heh, I've got a huge pile of them after Nick bought me a giant one in
a pot last year, and it seeded everywhere. It's a datura. The spikey
seed pods are really quite painful when you accidentally step on one!

The seeds, fwiw, are dimpled disk shape, kind of dark brown colour, I
think with a white blob at the dimpley side (kind of kidney shaped, I
guess). Can't think what dahlia seeds look like, so not sure how easy
they would be to mix up.