They are a welcome sight in my garden as well Nicola.
They self seed around the garden - sometimes in the right place!
For some reason, many self seeders here choose the very edge of my flower borders.
That one would look better in the mid border locations, but they get away with it where they choose most times
I have the double form and it is a very nice little plant and good for flower arrangements.
Do you have it?
It also self seeds. I haven't observed them enough to see if its possible to identify the single from the double forms before they actually flower. Does anyone know?