I don't know what brought this to mind tonight.
It was first shown in 1980, and repeated in 1982. I don't know which one I saw but I was either 12 or 14 when I saw it, and it stuck in my mind, although I could remember almost nothing of it.
So I looked for it, and found the whole 90 minute programme sliced up into 10 minute youtube sections, and I've just watched them all now.
They start with the one below and you should find links to next as each one finishes.
Conveniently, the next one starts by repeating the last few seconds of the previous one, which means there are no jarring little bits missing between the episodes.
I really enjoyed it. It's a simple enough science fiction story that is grounded not in big budget special effects, but in being a good story well told.
There's an excellent review (contains spoilers) of it at TV cream.