Mystery flower

One of the things about moving to a new house is the first year is spent discovering what's planted where. This flower has appeared in the front garden, and before the flowers opened it reminded me of an allium - couple of long green leaves, long green stalk, what looked like a single bud (which I was sure was going to be purple!). These flowers are all coming off a single stem. I have absolutely no idea what this is - any help identifying it would be gratefully received!