Trivia: Dorothea states that Ballon was hiding among the Zygons. The Zygons are an alien race with shapeshifting abilities that first appeared in "The Terror of the Zygons in the original Doctor Who, in 1975.
Ram and April enter the flat, and only Ram calls to Tanya. She then asks Jasper why Vivian didn't open the door to "them," but she has no way of knowing anyone else is there. She should have said "open the door to him."
Trivia: As Armitage tells Quill about the inspector, they pass a sign for the Barbara Wright Building of the academy. Barbara--played by Jacqueline Hill--was a history teacher at the school in 1963, and traveled with the First Doctor in the original series
Trivia: The series is set at Coal Hill Academy. The school was featured in 1963, in the original series premiere episode "An Unearthly Child" when the Doctor's granddaughter Susan Foreman (whose name is on the board of missing/deceased students) studied there. Since then, the Seventh, Eleventh, and Twelfth Doctors have visited it. Three of their companions--Ian Chesterton, Barbara Wright, and Clara Oswald--have taught there.
Trivia: The Doctor mentions that he used to be the caretaker at Coal Hill. This occurred in the new series, Season 8, in "The Caretaker."