The Summer Snowflake (Leucojum aestivum) flowers in spring despite its name. Confusingly there is also a Spring Snowflake (Leucojum vernum) which flowers when the name says it does. And they both look very similar (and both resemble Snowdrops).
 These are L. aestivum 'Gravetye Giant', a form selected from the wild by William Robinson around 150 years ago. The bulb trade seems to favour this variety as the most garden worthy.
 Robinson was a pioneer of naturalistic gardening and wrote 'The Wild Garden' which is still a very useful text on the subject. He developed his ideas and put them into practice at Gravetye Manor in West Sussex.