Mary Poppins

Mary Poppins was something the Eleventh Doctor wondered — in the midst of a discussion about his fitness as a babysitter —if the twelve-and-a-half-year-old Kazran Sardick had seen. The Doctor stopped himself before fully comparing himself to Mary Poppins, saying that the analogy would have been "rubbish". (DW: A Christmas Carol)

Though the Doctor never specifically called Mary Poppins a film, any common sense reading of the scene would indicate he was talking about the film — largely because of the line, "Have you ever seen Mary Poppins?"

When the Eleventh Doctor gathered magician props from the TARDIS, he placed them all in a large carpet bag which was bigger on the inside than the outside, stating that he was glad he got it back from Mary Poppins. (QR: Magic of the Angels)

This implies that Mary Poppins exists as a real person in the Doctor Who universe.