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.)