Tiny Tuesday 51 - Unidentified Flower.

This tiny little plant has appeared in an old hanging basket that has been out all winter.  The flower heads are just under a centimetre across.  I have no idea what it is, but there are some Sedum plants in the basket, so it may be a variety of that.  It would be great if someone could identify it for me.
Thanks go to Beckett, who is hosting Tiny Tuesday this week.