V

While under mental attack by the Mandragora Helix, Sarah Jane Smith, when reciting the alphabet backwards, mistakenly ordered the letter V after T and before U. (TV: The Masque of Mandragora)

While binding Chris Cwej's consciousness to the conceptual weapon V-Time, the Surgeon told Cwej that the letter V was, among other things, an ideal representation of the meeting of different points. (PROSE: The V Cwejes)

Behind the scenes

 * The placement of U and V is obviously a blooper by Sarah Jane's actor Elisabeth Sladen.