Small Copper (Lycaena phlaeas)
It is not a common species but widespread and is not unusual in suitable habitat in Dorset.
It is unusual in that it has three broods a year, possibly even four in hot years with an Indian summer. That means that you can see them any time from May right through to November. In good years there will be more adults flying from the later broods so the seem far more common in late summer.
The larvae feed on Sorrel and other species of Dock and the overwinter as a larvae which hibernates.
A lovely treat to behold when it opens its wings to soak up the warmth of the sun.