Thanks to Urban75 I now know that it was the former Priory Grove school, a site later developed as flats. It seems to have been occupied in 1991/92, with travellers living on site as well as puting on parties. Someone who lived there recalled that 'The redevelopment started shortly after the eviction, I used to live in the main building until shortly after the last party got busted and we lost electricity. Some great parties there, winter of 1991/2. Rave on the ground floor. Bands on the first floor. Fairshare reggae sound system on the top floor'.
Some photos and a flyer have surfaced recently on facebook, the latter from 'The Dynamic Pig Posse of Priory Grove' advertising an event in January 1992 with bands including festival/squat circuit favourites RDF and Co-creators as well as 'hardcore acid'. I know one of the parties I went to there was in December 1991, because I found a copy of a letter I wrote at that time (to a Brixton guy who was in prison for the poll tax riot) in which I said 'I went to a rave at a squatted school in Stockwell last week, which was good. Downstairs they had a serious rave, upstairs there were bands playing. There was also a massage room, although I didn't use it myself'.
(Images from Chris the Poet)