Mary Poppins

 was a fictional character dating back to 20th century earth. In a series of books and an iconic musical film of the 1960s, she was a magical nanny who arrived to help families in need. The film 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". (TV: A Christmas Carol)

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, saying he was glad he got it back from Mary Poppins. (PROSE: Magic of the Angels) Tanya Adeola compared a bag with a large assortments of various useful things to Mary Poppins handbag. (PROSE: The Stone House)

Andy Hansen was familiar with Mary Poppins and said if she tried flying in gale storms that were going on in 2015 she would end up in Norway. (PROSE: The Beast of Stalingrad)

The Sixth Doctor watched Mary Poppins on television one Christmas, during which he tried to fathom the aerodynamics of her umbrella. (PROSE: Christmas Special)

Behind the scenes

 * Though the Doctor never specifically called Mary Poppins a film, a 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?" The fact several other sources also refer to Mary Poppins as a film, the one-off reference to Mary in Magic of the Angels would appear more likely to be a joke than an actual indication that she exists in the Whoniverse.
 * Missy's appearance is based upon that of the Julie Andrews version of Mary Poppins, to the extent that she imitates the style of Mary's arrival (floating down from the sky using an umbrella) in TV: Death in Heaven.