Marbled White
Melanargia galathea

We are used to seeing these attractively-patterned butterflies around Weymouth and Portland, but they are not at all common in the UK away from the south-western counties.
Of course, the name is a misnomer as they are not "whites" at all but are in the family of brown butterflies along with Speckled Woods and Walls, which they resemble quite closely in pattern if not colour.

Why they are not found in other parts of the country is a bit of a mystery, as they will feed on most grasses as long as Red Fescue is one of them.