Night of the Demon is without doubt one of the greatest horror films ever made. It's creepy, it deals with black magic in a way that's effective and believable, the cast are awesome (including Peggy Cummins from my favourite film of all time Gun Crazy) and it's based on a MR James story. If you haven't seen it you should definitely come to the screening, because then you also get to see it with a group of like-minded horror fans which is always fun.
As for the campaign in general, I think anything we can do to raise awareness of classic cinema is important. When I was a student it felt like classic films were everywhere. Half of my degree was studying film so it was always there and always important. When I was being a proper student i.e. sitting on the sofa during all day smoking and drinking too much in an effort to cure my hangover from the night before there would always be some classic or other on TV to help take my mind off numerous existential crises. But at the moment I'm long out of university and don't have a TV aerial and while my Lovefilm list is dotted with films I feel I should've seen but never have those classic movies just don't seem as big a part of the world at large anymore. What I mean is, I rarely get into conversations at work or even with my filmmaking peers about old cinema. Sometimes someone might mention something from the 70s but generally that's about as far back as people go.
And then occasionally I meet a real connoisseur of classic cinema and am made to realise how few of the great films I've actually seen. Or I see something like Rififi (a recent example I saw for the first time recently) or Roman Holiday (which I watched again yesterday and always forget just how good it is) and I rave about them to everyone I meet like they're the hot new films that only came out last week. I'm going off topic here a bit, but my point is there are great treasures in the history of cinema and anything that celebrates that, like the Classic Horror Campaign, deserves to be supported. Which you can do by signing their petition and going along to their screening next week.