Hi Richard,
I believe this is actually a female Dun Skipper - I've come across several Dun's with white markings suggesting a Glassywing, but the lack of white at the base of the antennal clubs is usually always a clincher for me; in addition to the size/shape of the main white "glassy" spot. The illustration in Glassberg (Plate 62, #12) looks very similar to the individual above.