I can confirm there was something out of the ordinary going on here in the mid 80's. I was a Met Policeman at the time and we got called there to a dead body by an anonymous caller in the early hours of the morning. There was indeed a dead guy on the stairwell, I think it turned out he had fallen through a trapdoor in the roof to his death.
However...on searching one of the floors we found, by torchlight, which was pretty scary, a chamber that had been decorated. There was a bed, an altar and various restraining devices attached to the walls and pillars. Some of these were child sized. Only way to describe them was like body harnesses for pinning people up in the air. From memory it turned out the place was used by a group of squatters who had taken over a derelict terrace nearby for filming. Not the sort of film I would like to have seen. It was indeed a scary place that night, I've no idea what the result of the investigation was as I wasn't involved past that night. Could have been an orgy/film/black magic set, there was all sorts of weird stuff in the room.