It might be helpful to know how the family name of the owners of Petworth House has changed over the years. The first recorded owner of a house at Petworth is Agnes de Percy (d 1204). Percy is the long-established name of the Earls of Northumberland, the first of whom, Henry the 4th Lord Percy, was created in 1377. Petworth passed through the hands of 11 Earls of Northumberland, until Lady Elizabeth Percy (1667-1722) married Charles Seymour, 6th Duke of Somerset (1662-1748).
Their son was Algernon, 7th Duke of Somerset (1684-1750). In 1749 he was additionally created the 1st Earl of Northumberland and the 1st Earl of Egremont. His sister, Lady Katherine Seymour (d 1731) married Sir William Wyndham Bt. (1688-1740) and they had a son Charles Wyndham (1710-1763) who became the 2nd Earl of Egremont. Their grandson George Wyndham (1787-1869) was created the 1st Lord Leconfield in 1859. His grandson, Charles the 3rd Lord Leconfield, gave Petworth House to the National Trust in 1947.
His nephew, John Wyndham the 6th Lord Leconfield (1920-1972), was created the 1st Lord Egremont in 1963. The current seigneur is therefore correctly styled John Max Henry Scawen Wyndham, the 7th Baron Leconfield and the 2nd Baron Egremont.