"The Crimson Horror" is the only Doctor Who episode so far that features the Paternoster Gang but was not written by Steven Moffat. He brought in his Sherlock co-writer and fellow Victoriana obsessive Mark Gatiss to write his script while he was finishing off "The Snowmen," and because the two stories share a decent amount of their cast, they were filmed in a production block together.
Here are a few things to keep an eye out for, the next time you watch:
(The episode is available on iTunes and Amazon.)
There's an untold case attended by Sherlock Holmes that would fit the details of this story rather well. It's mentioned in passing by Dr. Watson in "The Adventure of the Golden Pince-nez," who, while rifling through his case files, says, "As I turn over the pages I see my notes upon the repulsive story of the red leech and the terrible death of Crosby the banker."
The community of Sweetville is based on the real-world working villages of Saltaire and Akroydon, founded in the 1850s by textile industrialists Titus Salt and Edward Akroyd. Although it's a short conceptual hop from Salt to Sweet, Gatiss was inspired to name his villain Mr. Sweet after a discussion about Victorian life with fellow Doctor Who fan Matthew Sweet.
Rachael Stirling (who plays Ada) was recruited to the cast after she appeared with Gatiss in a production of George Farquhar's Restoration comedy The Recruiting Officer at the Donmar Warehouse in London in 2012. Gatiss already knew Rachael's mother, Dame Diana Rigg, and the three had dinner one evening while Gatiss tried to convince them to play a mother and daughter on screen for the first time. He had to convince Diana with a showreel of classic Doctor Who villains, as she had never seen the show. Oddly enough, Gatiss' co-star in The Recruiting Officer was The Office star Mackenzie Crook, and he also recruited Diana and Rachael to play mother and daughter when he wrote his acclaimed BBC comedy Detectorists.
Speaking of Diana Rigg, Jenny is seen fighting off some of Sweetville's workforce while wearing a leather cat suit, which was inspired largely by Diana's costume in the 1960s TV adventure series The Avengers:
Although it was filmed in Cardiff, Gatiss wanted to write a Doctor Who adventure set in the North of England, using northern actors (including Dame Diana Rigg, who is originally from Doncaster). He originally titled his story "Mother's Ruin," a play on the eighteenth-century slang term for gin.
When the Doctor says he once spent a "long time trying to get a gobby Australian to Heathrow Airport," he's referring to the story arc of Tegan Jovanka, companion of his fifth incarnation and cabin crew worker who managed to wander aboard the TARDIS for the ride of a lifetime. The Fifth Doctor often encouraged her to buck up and stop complaining with the catchphrase "brave heart, Tegan," which is why he then goes on to say the same thing to Clara.
The Fourth Doctor had some experience of accessing an image of a recently deceased being, although in his case, it was a Wirrn who died, and he was accessing its retina in "The Ark in Space."
Mark Gatiss worked in references to several foodstuffs that will have been familiar to people in the 19th century, including Pontefract cakes (also known as Pomfret cakes and Pomfrey cakes) which are not cakes at all. They're a type of small circular sweet made of liquorice, and were originally manufactured in the Yorkshire town of Pontefract. Mrs. Gillyflower also mentions seed cake: a traditional British cake flavored with caraway seeds that appears in British recipe books from as far back as 1591. The most notable of Victorian cookbooks — Isabella Beeton's Book of Household Management — also has a seed cake recipe.
Fans of Victorian gothic horror will have already spotted that Mrs. Gillyflower's offering of "a glass of Amontillado" sherry is a nod to Edgar Allen Poe's short story "The Cask of Amontillado," in which a murderous nobleman tempts a rival into a cavernous cellar on the promise of some rare sherry, only to chain him up in an alcove and seal him in with a brick wall.
And as for "trouble at mill!," it's an archaic idiom from the North of England (hence the delight with which the Doctor says it in a Yorkshire accent) dating from the Industrial Revolution. It's based on situations of industrial tragedy and strife, which later came to take on a wider meaning, that of non-domestic aggravation happening in a workplace or community environment:
Monty Python's Flying Circus later began their celebrated Spanish Inquisition sketch with the same phrase:
Now go back and read the entire 10 Things You May Not Know About Doctor Who archive.