We call it lady's smock or cuckoo flower - as it is meant to flower at the same time as the cuckoo arrives (sadly don't get some many cuckoos now to put that to the test). It grows wild in some of the verges round here. I love it, particularly because as Tristan says, it is good for the orange tip butterflies. I've just bought a plant of the double form from Rumbling Bridge Nursery (along with Anemone sylvestris ‘Flore Pleno’, another good cottage garden plant).