Unknown flower - NOT amateur radio

This flower self-seeded in our front garden. We did not plant it. The flowers are about 3cms across. It is probably quite a common sort, but this is the first year we have seen it. We have been here almost 5 years now. It is May flowering. Some have died and just this one is in flower.

UPDATE 1340z: Someone told me it is "Star of Bethlehem".