It's Bigger on the Inside (in-universe)

 was a book that had been published by 1974. Somebody walking outside the BBC Television Centre was reading it when a plethora of scripts were thrown out of a window because of their redundancy now that Tom Baker was the new Doctor. (NOTVALID: Ever Wondered What Happened At.. The Auditions For The Seven Doctors?)

Behind the scenes
The title of the book, referencing the Tim Quinn and Dicky Howett book It's Bigger on the Inside!, makes an appearance here in a similar vein to the cameos of The Doctor Who Fun Book which also referenced an earlier work by the duo.