I didn't see this back in the Seventies, but bought the DVD on the strength of the reviews here and elsewhere. I'm glad I did; it's a thoroughly enjoyable story with some nicely scary moments.
I disagree with the person who complained about "just how bad children's tv drama from the 70's could be". Nope; a lot of it was actually very good -- think of the individual stories in The Shadows (1975-78), or Alan Garner's The Owl Service from the beginning of the decade, or his terrifying Red Shift from 1978, the same year as The Clifton House Mystery. This certainly isn't faultless -- the children tend to go to their spot and project their perfectly-rehearsed lines, and the parents are a tad stereotyped in their roles -- but for me that didn't spoil the enjoyment at all. The only thing that I found irritating was the music box/sig tune; once it got in my head it was difficult to shift.
What I found amusing, and very much of its time, was just how middle-class the children were, when today we're used to all kids on TV, even those from professional families, being streetwise and a bit in-yer-face. I don't think the kids from Seventies' TV series exist today -- but perhaps they never did.