This film, part of the Cult strand at BFI Southbank, also screens on 19th April. You can find all the details here.
Here is the BFI introduction:
The spirit of William Castle haunts this affectionate B-movie tribute, in which an all-night horror marathon at a dilapidated movie theatre becomes a veritable bloodbath for a gang of ill-fated film students. This curious precursor to the self-reflexive horrors that flooded 90s genre cinema is hugely fun and highly inventive.
Here (and above) is the trailer.