 When the 8th Marquess died in 1955, Alistair became the 9th Marquess at age 18.He renovated the family estate, Wynyard Park, County Durham, which Nikolaus Pevsner described as "the most splendid 19th century mansion house in the county".Wynyard was sold by Lord Londonderry in 1987 and is now owned by the property developer Sir John Hall.