C went upstairs when I pressed 'play' on this, afraid that it would turn gruesome or grotesque in that victimized-vacationers-Hostel way. But of course it didn't, because it's British, where characters knit jumpers and go to the train museum.
That said, they do reveal themselves to be lethal and damaged and generally unhinged, with the mayhem coming on slow, then jagged and almost casual, and all of it undermined by a kind of hopelessness, an adventure story of the doomed (and probably always doomed).
On Netflix. Just press 'play'.