Mother Shipton (Callistege mi)
Butterflies fly by day and moths by night? Far from an accurate way of separating the two I'm afraid. There are several moths that only fly by day and this one, Mother Shipton, is one of them.
It is called Mother Shipton because if you look at the dark patch on each fore wing and use your imagination you can see the face of an old hag - long nose, pointed chin, black eye; can you see it?
Mother Shipton is one of two common day flying moths that you can see on downland and other grassy places across Dorset in May and June, the other is the Burnet Companion which is similar.
Easily mistaken for a butterfly until you see it at rest like this and then it looks just like ... a butterfly! However, it is a moth, all the books say so.