A BBC Radio 4 program last year — and just rebroadcast last week — goes into some detail about why the Doctor Who story “Shada” was never broadcast, and more:
Shaun Ley recounts how the political circumstances of the late 1970s resulted in three of the most popular TV series’ of the time – Dr Who, Secret Army and The Professionals – each having at least one episode scrapped after filming. Interviewees include the then Dr Who, Tom Baker.
You can listen online — even in the U.S.! — here, but only till this evening.
(Thanks to John for the link. If you stumble across a cool Doctor Who thing, feel free to email me with a link.)