I lived in the ballroom at Stonelands from 1972-1979, then at Bullards and Brooke House until 1986, mostly Bullards. Bullards was gorgeous when I first moved there in 1979. It was still not bad in 1986 when I moved to LA, although not quite as nice as it had been. At the time is was berthing only, mostly AOSHUK. I heard it deteriorated later on. Brooke House was pretty much always a pit, and (accidentally) burning it down improved it remarkably.
Stonelands was kind of OK in the 70s, if
freezing, apart from its inconvenient location. It certainly had character. After 1979 when pretty much all the single people moved out to Bullards and Brooke House, leaving the place to the gangs (mobs of unsupervised kids running amok) during the day, the property deteriorated fast, with lots of windows getting broken and not repaired, and so forth.
I never heard about a secret passage behind a fireplace. Still, the original building dates from c. 1580 (remember that winding staircase next to the kitchen?), so who knows? 
Paul