A pretty flower which has volunteered

I'm not sure where this plant came from. I must have bought one in the last year or two, but now there are four separate plants growing in an old pot on the patio. I only spotted it yesterday when watering the pots. It was nestling in a bunch of weeds. Now I've found it again I will nurture it and try to gather some seeds, just in case.

What a lovely mid-summer's day!