I saw this episode a few years back not having seen it since it originally aired. It still has a creepy air about it, but I found it was more an allegory.
In the final scene Frances Anne has physically matured and returns to the cairn where the townspeople had trapped the creature and she promises to release it. Faintly you can hear the creature stirring inside.(Though how they managed to keep the monster still whilst entombing it is anybody's guess).
It seemed to represent Frances Anne's sexual awakening or coming of age. Her father seemed particularly intent on keeping this creature trapped and was not pleased upon learning of his daughter's benevolent relationship with it.
Although not the series' best episode there is an uneasy atmosphere throughout and Pam Ferdin delivers a fine performance. (Later in her career she would play Felix's daughter Edna on the Odd Couple as well as voicing the character of "Lucy" in the Peanuts cartoons.)
Well worth another look if you get the oppurtunity.