Roy Precious - Antiques & Fine Art has dated this piece to the 1700s.
The sitter is thought to be the Duchess of Somerset; c.f.
Illustrated on page 260 of the National Portrait Gallery publication 'Later Stuart Portraits' by John Ingamells.

ElLIZABETH, Duchess of Somerset (26 January 1667 – 24 November 1722), a major heiress, was born Lady Elizabeth Percy, the only surviving child of the 11th Earl of Northumberland and deemed Baroness Percy in her own right. She carried the earldom of Northumberland to her son Algernon. Lady Elizabeth was one of the closest personal friends of Queen Anne.

Marriages:-
She married firstly, Henry Cavendish, Earl of Ogle, the heir of the 2nd Duke of Newcastle on 27 March 1679, but he died the following year.
She married secondly, Thomas Thynne, "Tom of Ten Thousand" due to his great wealth, a relative of the 1st Viscount Weymouth, on 15 November 1681, but he was murdered the following February by Swedish Count Karl Johann von Königsmark using a gang after gossip said her marriage was unhappy and Königsmark began to pursue her.

Five months after the death of Thomas Thynne she married on 30 May 1682 Charles Seymour, 6th Duke of Somerset, known as "The Proud Duke", and so became Duchess of Somerset. She brought him immense estates, including Alnwick Castle, Petworth House, Syon House and Northumberland House in London.

She was Mistress of the Robes to Queen Anne from 1710 to 1714.
Lady Elizabeth had four children:
Algernon Seymour, 7th Duke of Somerset (11 November 1684 – 7 February 1749)
Lady Elizabeth Seymour (1685 – 2 April 1734)
Lady Catherine Seymour (1693 – 9 April 1731)
Lady Anne Seymour (1709 – 27 November 1722)
MICHAEL DAHL (1659 - 1743).
Dahl was a painter of exceptional talent and regarded as the only really serious rival to Sir Godfrey Kneller, for royal patronage, during the years 1690-1714. Dahl's patterns were undoubtedly indebted to the fashion set by Kneller, but Dahl had a lighter palette, his brushwork applied in shorter and more careful strokes.
His self portrait hangs in the National Portrait Gallery and he is famed for having painted a series of wonderful female portraits for the Duke of Somerset, now at Petworth House, and known as the Petworth Beauties.
Dahl's portraits of members of the royal family hang at Kensington Palace and Windsor and other examples of his work can be found at the Tate and National Maritime Museum, Greenwich.
SIZE: 57 x 48 inches inc. frame.
PROVENANCE:
Private Collection, Sussex.
Collection of a Fellow of a Cambridge college.
Roy Precious - Antiques & Fine Art has dated this piece to the 1700s.
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