The females are difficult.  The best technique is to find a nice area of sallows, particularly stands of Goat Willow Salix caprea that are partly shaded by taller trees, and heft yourself there for ages.  Females are most active between 12.00-3.00, but are less active than the males.  The females are not beginner’s butterflies. 

Telling the sexes apart is not easy.  The females are larger, have broader hind wings, less sharply angled forewings, and the white band on the hind wing underside is broader.