Her article in thepeerage is here. I have no idea who the artist is. "Daconman" may be a Deviant Art contributor.
From goodgentlewoman.wordpress.com/2012/05/10/lady-jane-st-john-mildmay; other images from portraits of her painted in the late 1700s can be seen at this site.
An older Lady St. John Mildmay wears a dress with wide shoulders covered by large epaulettes beneath a capelet and bonnet.
Keywords: Jane Mildmay, Jane St. John, Mildmay family, St. John-Mildmay family, British, curly coiffure, ribboned bonnet, bows, frills, strap, capelet, lace, high neckline, coat, epaulettes, gigot sleeves, belt, natural waistline, necklace