Celebrated for his work in pastels and oils, Cotes was a founder member of the Royal Academy of Arts, rivalling Joshua Reynolds and Thomas Gainsborough in portrait painting before his untimely death aged 44.
The artist exhibited 18 portraits in two years at the Royal Academy’s annual exhibitions, before dying from the effects of a toxic potion he had taken in an attempt to cure his kidney stones.
The undated portrait at Sworders depicts the wife of Sir James Winter Lake, Joyce Lake as Diana in a white dress holding a bow with a dog by her side.
Consigned from a private vendor in Norfolk, it was secured by the London trade above estimate at £6800.
Elsewhere, a later work by Edwardian painter William Ranken (1881-1941) sold for £2500 against a £700-1000 guide. The signed 3ft 1in x 4ft (94cm x 1.23m) oil on canvas, dated 99 and titled On the Bow of the Gin Palace, sold to a private collector in Scotland.
The sale took place on September 11.