It has a genuine folksy weirdness to it that charmingly verges upon the camp, but never quite topples into being truly ludicrous due to its erudite script. It has that sun-drenched '70s cinematography in spades, despite being a television piece on a clearly limited budget. Somehow greater than the sum of its parts. Despite being hokum, it convinces the viewer through its rootedness in the landscape. Sly and curious.
A well modulated exercise in mild scares - it originated as a television programme for children - that manages to tap into fairly erudite topics concerning ancient rites and mythology. As noted elsewhere, you'd be hard pressed to see this type of thing made for adults, let alone children, on modern British television unless the allusions were swapped out for much running around the stones instead.