This is a real long-shot: in an article by Alan J Miller in Dorset Life, Apr 2003 “Heartless Landlord and Devoted Husband”, there is an image of a young Joseph Damer, by permission of LG Stopford Sackville.
I understand that Drayton House is now owned by Charles Lionel Stopford Sackville. Does anyone know how we could contact them for more information – such as where is the painting, is there any other information on Joseph Damer, Lord Milton?