Q

Q was the seventeenth letter of the alphabet, following P and preceding R. (TV: Shada, AUDIO: Shada)

When trying to remember "Young Parsons'" name, Professor Chronotis went through the alphabet with the Doctor and Romana II. They started with A, B, and C and eventually got to P, Q, and R before he skipped to X and Y. (TV: Shada, AUDIO: Shada)