U
From Tardis Wiki, the free Doctor Who reference
U was a letter in the alphabet, which, by most accounts, succeeded T and preceded V. (PROSE: The Dalek Dictionary, The Book of the War [+]Loading...["The Book of the War (novel)"], etc.)
When Sarah Jane Smith recited the alphabet backwards while under mental attack by the Mandragora Helix, she placed U after V; (TV: The Masque of Mandragora [+]Loading...["The Masque of Mandragora (TV story)"]) however, in another account of these events, she ordered V after U. (PROSE: Doctor Who and the Masque of Mandragora [+]Loading...["Doctor Who and the Masque of Mandragora (novelisation)"])