Her article in thepeerage is here.
From bbc.co; image enlarged by half.
Lady Arabella wears a cap topped with prominent feathers and a British casual draped skirt in this Hoppner portrait.
Keywords: Hoppner, Arabella Diana Cope, Duchess of Dorset, Cope family, Sackville family, Duchess, British, curly coiffure, ribboned feathered cap, U neckline, long close sleeves, cuffs, waist band, close skirt, drapery, shoes, wrap