Miss Higgins says that she's feeling at sixes and sevens. If you are at sixes and sevens you are in a state of total confusion, disorder or disarray.
The opening credits includes Sister Hilda waking up next to an open copy of an Agatha Christie novel. Finella Woolgar, the actress playing Sister Hilda portrayed Agatha Christie in the Season 4, Episode 7, episode of Doctor Who "The Unicorn and the Wasp".