Beautiful - but what is it?

Dull overcast morning - but still getting warmer.

I discovered that in Blipworld, this is Tiny Tuesday. Since I have to go out today for a session of Children's Hearings I looked to the garden for inspiration.

This beautiful little plant has petal heads which are only one inch (2.54 cm) across in total. It has flowered each year since we moved here four years ago but, since it was planted by the previous owners, we have no idea what it is called. If anyone out there knows, perhaps you would let me know.