Dr Who was also filmed at Graig Ddu slate quarry (episode Five Doctors) another episode was filmed at the Brockham Limeworks, near Dorking, Surrey (at that time housing, and making use of Brockham Narrow gauge Railway Museum).
Artemis 81 starring Hywel Bennett and Sting was filmed in some of the tunnels of Dinorwic Power Station.
Treasure Hunt with Aneka Rice visited Llechwedd during one program.
Brave Old World (an HTV documentary with the late Wynford Vaughn-Thomas) visited Maen Offeren, Rhos, Penrhyn, Oakeley, slate quarries and others.
Slate Makers (a BBC2 documentary) visited Pen yr Orsedd, Maen Offeren, and other slate quarries.
Another TV series used Garreg Fawr/Silurian Iron mines. Sorry, I can't remember much more I think it was a BBC programme and was based on Merlin filmed late 80s or early 90s.