Unknown flower

I had a wander round the garden when got home from work today, when I noticed this pretty flower. It comes up at this time every year and only lasts a little while but I don't know what it is, it was already in the garden when we moved here 10 years ago. It's lovely to see such a delicate little flower at this time of year, a sign Spring is here, would like just a little bit of warmth now though.