Streamer.
(Anticlea derivata)

For almost a month I couldn't make my mind up and then, one day, I saw it in a book.

Information:
Once I saw a picture of it, the "Streamer" from which it gets its name is the swirling pennant flying in the tip of each fore-wing, it was obvious. The pair of thick black-to-grey cross lines enclosing a broad zig-zagged bar and the general grey colour are distinctive. This particular one even has the violet tinge to the grey that denotes it is freshly emerged.
They fly April-May and can be seen in gardens, hedges, open woodland and rides or at the edges of woods. Their Fore-wing length is 14-16mm and they are fairly common throughout Britain.