This is one of several moth species that only fly by day. It is called Mother Shipton (Callistege mi) but why on earth would a moth be called Mother Shipton? You need to look very closely at the dark patch on each fore wing, use your imagination and you will see the face of an old hag - long nose, pointed chin, black eye; can you see it? The original Mother Shipton was a 16th century witch!
Mother Shipton is one of two common day flying moths that you can see on downland and other grassy places across Dorset from May until July, the other is the burnet companion which is similar in many ways so care needs to be taken. Flying by day they can easily be mistaken for a butterfly, especially the dingy skipper, until you see them at rest like this and then they look just like ... a butterfly!
The larvae feed mainly on plants of the pea family and so areas rich in clovers, vetches and the like are the most likely places to find the adults. They are very active insects, always alert and sensitive to movement, so you need to be very careful in your approach.