This is what is known as Ramsons, or more commonly "wild garlic", in the UK and Ireland. It is Allium ursinum, a different species from the North American Allium tricoccum, although judging by the photographs they look very similar. Four years ago I planted a patch about 4 ft x 3ft with bulbs and it is now growing very thickly, and has spread by seed into the adjoining gravel path and is growing in gaps between paving slabs. This little patch provides more than enough for us to use in salads and a variety of dishes.