From the particulars: "The ruinous Abbey house, which sits adjacent to the old site of The Abbey, was rented by Sir Francis Dashwood, afterwards Lord Le de Spencer, and in the early 1700s was restored for him by Italian artists. It was here that he founded the famous brotherhood of ‘the Knights of St Francis of Wycombe’ in 1745 with the motto of love and friendship. This profligate society had the dress consisting of a gown and a turban of crimson and blue satin, with the device in silver. The society was limited to 24 members of rank and fashion, mostly resident in the neighbourhood, who resorted to Medmenham Abbey during the summer months. Over the main entrance was placed the famous inscription ‘Fay ce que voudras’ or do what you will." The inscription also appears above one of the fireplaces pictured in the brochure.
I think the cinema (with tiered seating) post-dates Dashwood.
It is obviously a rather desirable house in many ways. But if you look at the map at the end of the particulars, you will see that it has an almost immediately adjoining neighbour to the west, despite having acres of parkland and privacy to the east - something to be aware of if you are thinking of re-founding the Hellfire Club.