If you ask me, 'Horror of Fang Rock' is one of the best ever; a thriller that focuses on characters who really interact while they're trapped together, featuring Tom and Louise at their acme.
It investigates the nature of belief in an age of rising science and technology: Adelaide's astrology fetish compares to the superstition of Vince and Reuben, with Vince's terror as real as hers, and Reuben's fear of monsters more a manifestation of melancholy stubbornness at the rise of unsympathetic forces he doesn't understand (like electricity... which is also the weapon of the monster that kills and impersonates him). Meanwhile, Leela lectures Adelaide that consulting her "shaman" (despite Adelaide's denial, that is the right word for people like Miss Nethercote) is a "waste of time"... but, with relishable irony, the semi-educated Leela simply believes in science because her mentor has told her to.
'Fang Rock' has a quiet undercurrent about sex too. Adelaide is understood by Harker to be Palmerdale's "fancy woman" and Skinsale obviously envies this (though god knows why)... but he's also clearly very taken with Leela. Paddy Russell gives us a whole shot simply to establish how much Skinsale digs Leela on sight. Vince is very flustered by Leela's impromptu striptease (which she completely fails to understand) and pays attentions to Adelaide, which she simply treats with a patronising indulgence (to her he's an instrumentum vocale, not a young man). And then we have the porn under Reuben's bed. Mind you, I'm not a Freudian so I'm not saying anything about phallic symbolism. Sometimes, a lighthouse is just a lighthouse.
'Fang Rock' is also very interested in class and the way it was changing in the early 20th century. Palmerdale is loaded and is therefore a scumbag, obviously. Crude but very Whoish... and true enough in general terms. He seems dead posh to the keepers... but to Skinsale (a wonderfully apt name, considering the manner of his death) Palmerdale is a nouveu riche, arriviste, vulgar little money-grubber... and such things obviously matter to Palmerdale since he has evidently purchased his lordly title. He tries to buy everything. He even seems to be paying his mistress (albeit for being a secretary rather than a concubine). Skinsale, meanwhile, considers himself Palmerdale's social superior, despite obviously being skint (old money, obviously... and with a reputation built on enforcing empire in India). Palmerdale gets loads of his sailors killed but makes sure that he escapes (there are lifeboats enough for the gentry, as with the Titanic disaster, but not for the commoners) after causing the crash because he's anxious to use insider info to fleece the markets... and then views Harker's recriminations as irrelevant impertinence, and the keepers as more servants.
It's worth noticing that the Doctor treats the workers (other than the cloddish and xenophobic Reuben) with respect. He starts off calling Vince "Mr Hawkins" while the poshos automatically talk to him like he's a footman.
Vince, upon realising that Palmerdale is dead, burns the cash that His Lordship gave him, instinctively understanding that if he's found in possession of it, the society he lives in will simply assume that he murdered the venal, titled git for the money.
Perhaps you think I'm imagining that this story is obsessed with class, status and snobbery? I ask you to remember that seemingly throwaway detail that the Rutan refers to the Sontarans as "rabble". See what I mean?
Okay, it ain't revolutionary. Very little Who is. 'Fang Rock' offers a lefty/liberal critique rather than a radical challenge. But the critique is quite strong and sophisticated.
It does occur to me that the Rutan embodies electricity and technology and militarism... and the oncoming 'Great War' was pretty indiscriminate in the way it killed, precisely because of the startling new military technology that it produced.
It's interesting to ponder what China Miéville calls "the tentacular revolution", that strange occurrence within sci-fi/horror fiction, roughly starting round about the turn of the century, in which the West suddenly discovered the uncanny potential of the giant squid and the octopus. The tentacled beast also saturates political posters and propaganda round about this period (reaching a crescendo in the twenty years or so around WWI). Have a look at this amazing blog.
Miéville doesn't give a straightforward or reductionist reading of this tentacular trend. He suggests it represents a failure of meaning (the octopus is amorphous and terrifyingly unlike anything else specific, hence it's like everything) in an age of uncertainty. But it seems to me that it can also be connected to the rise of modern communication and travel (railways, telegraphs, etc) and to the rise of global companies, global military aims and political movements. Which is precisely the sort of thing the Rutan embodies - an imperialist philosophy, communication (his signal modulator), new weaponry, new tech, etc - and which the other characters talk about too (i.e. Palmerdale with the wireless telegraph, etc).
The most salient physical feature of the octopus/squid is its many arms - which is graphically perfect for representing 'global reach' of the kind attributed to countries, companies, ideas, technology, imperialist armies, etc... especially in an age of expansion (or, to use our terms, globalization). In political posters, the octopus is constantly represented hugging a map or globe, with its arms in many places.
Of course, 'Fang Rock' was made in the 70s not the 1910s, so it doesn't have quite the same meaning as octopus horror stories by H.G. Wells or Tsarist anti-semitic octopus posters... so this is a case of semiotic drift. The signs and symbols of one age get a piggyback ride on its representation in a period drama. But there were similar things in the air in the 70s too, thanks to Vietnam, etc. And now also, which might be why the tentacle is making something of a comeback in political posters (often with either 'al-Qaeda' or 'Neo-liberalism' written on it, depending on who produced it) and also in modern fantasy writing.