Factual error: Billy Claiborne, the only member of the Clanton gang to survive the O.K. Corrall shoot-out, is conspicuous by his total absence in this story. Maybe that's how he survived?
Revealing mistake: The "American" accents in this episode are all over the map and frequently inconsistent within the same scene.
Continuity mistake: The six-shooters, in the climactic gunfight, fire off a lot more than six shots without reloading.
Visible crew/equipment: In the fourth part, as the mortally wounded fighter is falling against the back wheel of the cart, the camera is positioned low and its shot is panned upwards, revealing the top of the cyclorama, a large concave backcloth that has been pulled around the studio walls (presumably BBC Studio 1) to give the impression of daylight in these studio-shot but supposedly external scenes.
Other mistake: In part one, one of the gunmen shoots at a bottle behind the bar and it takes several seconds for the bullet to travel only a few feet.
Answer: Your right it is, "Island of Terror."