R

As spoken by Mr Smith, R (Morse code: .-. (TV: The Man Who Never Was)) was the eighteenth letter of the alphabet, preceeding S. (TV: 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)