Stuart Sutcliffe (23 June 1940-10 April 1962) was a musician who left the Beatles in Hamburg, according to Jack Harkness, "before they made it big".
Jack called him a nice guy, stating that he invented the mop-top haircut and was a dab hand at the old collage technique. Jack was confused as to whether his girlfriend was called Astrid or Ingrid.
Jack named one of the alien tapeworms Stuart in his honour. (PROSE: Slow Decay)