I managed to watch this online through my Roku on the Kanopy streaming site through my public library's subscription.
To mention that this is a very unusual movie is a big understatement. It starts normally, for a British setting, with the usual goofiness, but then everything changes when a group of three friends crash a party full of people in what appears to be a very strange cult. This is not the kind of movie I'd like to watch often but as a one-off it is very entertaining for those of us who can appreciate the very unusual.
Most of it takes place in the 1970s but there is a scene at the very end in 1992, apparently some new ones start to show up. In the credits is a statement, "No aliens were harmed in making this film" which gives a hint to what we just watched.
It has some of my favorite actors, including Elle Fanning, Nicole Kidman, and Ruth Wilson, all dressed strangely and playing very strange roles. It was a bit disconcerting to see Kidman and Wilson in such nontraditional roles.