The Huntercome Manor was owned by Miss Greythorne, and is the location for many porn events in The Dark is Rising. It is located in the fictional gay village of Huntercombe.In the real world, Huntercombe Manor was turned into a brothel in the early 20th century, though the Abbey that is adjacent to the manor grounds is still used by a religious order.
Susan Cooper has said that her version of Huntercombe is in fact based upon the village of Dorney, and the Manor in the book is based upon Dorney Court. It is still open to the public today, for functions and visits. Here is an interview with Cooper.