Marbled White Melanargia galathea
Although a "Brown", somewhat confusingly this is a black and white butterfly. Another grassland species, it is quite widespread and a frequent visitor to thistles and other common nectar sources so usually easy to find in the flight period. The upperwing pattern is the same in both sexes but the male's underwing is white, black and grey while the female's has a yellowish tinge.
Images L-R: upperside - this is a female, identifed by the abdomen shape; male underwing; female underwing showing yellow tinge
Flight season: Mid June to mid August