This is episode 3 of the 1979 series The Quatermass Conclusion (released full-length as simply Quatermass), and I can't seem to find the other parts online quite yet. But there's enough to chew on as Quatermass discovers a UFO cult in the British countryside that's being manipulated for sinister purposes.
I'm stunned - again - by Kneale's meticulous attention to detail, keen understanding of human behavior, and most of all- his prescience.