Whatever the complaint was about Miri the BBC took it incredibly seriously.
Miri was broadcast on 2/12/1970. The Empath was due to be shown two weeks later on 16/12/1970 but pulled at such short notice that newspaper listings and the Radio Times had already gone to print. Sites like BBC Genome still have The Empath listed as being shown at 19.20 but the limited access I've had to BBC documentation confirms that The Paradise Syndrome was shown instead.
What's interesting is that it looks as if The Return Of The Archons was also temporarily caught up in whatever happened to Miri, The Empath, Plato's Stepchildren and Whom Gods Destroy . It gets shown once in 1969 and then doesn't get shown again until 1976; when it might have been expected to be repeated in 1971 and 1974.
This next bit is speculation but when The Return of The Archons does finally get shown again it seems to have been hacked about. The programme as broadcast information for the 1969 première give a 16mm print length (yes the BBC records include film print length) of 4510 feet (if you assume that the 16mm bit is wrong and it should be 35mm then this is a running time of around 50 minutes. The film length for the 35mm 1976 repeat is 3837 feet (a running time of 42 minutes). There's no way to confirm this but I wonder if the Festival sequence was removed or shortened? It must have been weird to be a Star Trek fan in 1976, you'd have seen pretty much every episode twice and then suddenly the BBC shows what seems to be a brand new story.
I'm not sure if any details of the Miri complaint have been kept by the BBC, because it wasn't a BBC production. So that information might be gone forever -unless anyone else knows differently.
Fascinating stuff - thanks. It's possible they had 16mm prints, isn't it? Weren't 16mm prints sent to some affiliates in the US? I seem to remember reading that somewhere. Maybe the UK received them, too.