Why is there no video footage online of Black Sabbath's 1978 TV performance on Look, Hear? (see details before replying)?

For fans or people who are familiar with what I am talking about only please, I don't want any conjecture from people who aren't familiar with this specific thing.

It's well known by Sabbath fans that in 1978 the band did a one-off performance on the TV program "Look, Hear" with Dave Walker on vocals. It was their only performance with Walker. However, there seems to be no footage of it on the internet anywhere! There is audio of it, but no video.

This seems odd to me; we have access to things like footage of Jimi Hendrix playing in his high school auditorium but no footage of this major televised performance?

It has been confirmed that the footage DOES still exist, as it was screened a few years ago at an event in London that featured several other rare performances, so it isn't lost, and obviously there were people who captured it from the fact that the audio exists online.

Does anyone know if video of this performance exists anywhere online, and if not, why it doesn't?