W

As spoken by Sarah Jane Smith, W was the twenty-third letter of the alphabet, following U and preceeding X. (TV: The Masque of Mandragora, Goodbye, Sarah Jane Smith)

While unable to directly oppose Ruby White, Mr Smith told Clyde Langer "letter two (B), letter five (E), letter twenty three (W)", which Clyde interpreted as "beware". Mr Smith then added "letter eighteen (R)", with Clyde deducing that he was trying to warn him about Ruby. (TV: Goodbye, Sarah Jane Smith)

Behind the scenes
In the real world, W immediately follows V.