I saw During Barty's Party when it was first shown on British TV in 1976; I was twelve and it was a special treat from my grandparents, whose house I was staying at. I had nightmares for weeks! This is a wonderfully crafted story: two actors and the suggestion of a terrible, unseen threat worked together in scaring the undergarments off this viewer! Some of the images, notably the last one, have stayed with me for 30 years. Do NOT watch this alone.
Without giving too much away: as the story unfolds, everything works by suggestion. Washed-out, bleak 70's British TV production values help in establishing the normality that horror needs as a baseline for all the nastiness to happen against. And the ending is truly haunting.