I would think that the two obvious contenders would be either:
The Stars Look Down (1974)
How Green Was My Valley (1976)
both BBC adaptations that had previously been made into films in 1940.
I have another vague recollection of a Sunday evening short drama serial from the late 70's.
As I recall it was set around a Welsh mining community and I remember it featured a gravity railway system from the top of the mountain.
I may be mixing up a couple of programmes, not sure.
I would think that the two obvious contenders would be either:
The Stars Look Down (1974)
How Green Was My Valley (1976)
both BBC adaptations that had previously been made into films in 1940.
I think this may have been aimed at older children, if that helps.
I remember watching the stars look down(BBC version) with my dad. Pretty sure it was in the evening and aimed at adults. I remember some miners being trapped in a tunnel collapse and all slowly suffocating. I was pretty devastated by it and remember wanting to cry.
Sam
Not set in Wales and not a short drama but it was about coal mining. As you say - maybe a few programmes getting mixed up.