Ghostwatch and the real stories it drew from, by documentarian Adam Curtis.
Thanks to Stephen Collins for the link.
I was there that night, Hallowe’en 1992, and I’d read the Radio Times in advance and saw the Screen One ident so I knew what was up, and could only watch it purely as drama. I like to think I would have twigged before the end.
(I also borrowed some footage from it for one of my planned Doctor Who PBP episodes, based on the unmade Mark Gatiss script about the Doctor landing in a live haunting broadcast. Naturally I ended the trailer with an ominous Please Stand By card.)