Maria Skerritt was Prime Minister Robert Walpole's mistress and then his second wife. Her Wikipedia article is here. Her article in thepeerage is here. She died due to a miscarriage the year she married Sir Robert Walpole, 1738.
From stepneyrobarts.co.uk/7215; the original image is several degrees off orthogonal so I turned it with Photoshop. That left two triangles of bare space, a larger one below her skirt to the right and a small one to the left of her skirt bottom. I filled these in and also removed spots throughout the image with Photoshop.