Alan Titchmarsh, with celebrity experts and enthusiasts, reveals the hidden treasures to be found at Britain's historic houses and estates. Alan visits Quarry Bank Mill in Cheshire, an 18th-century cotton mill that is still home to a working mill as well as a museum. Oz Clarke uncovers some Civil War history at Dunster Castle in Somerset. Jon Culshaw visits a stately pile in an unusual location, Dan Jones joins an architectural dig on the south coast, and Clive Aslet visits the spectacular grounds of Biddulph Grange in Staffordshire.