John Francis Christian Ducker, (DWM 498) better known by his stage name John Leeson, voiced K9 from 1977-1979 and again from 1980-2010 on Doctor Who, K9 and Company, The Sarah Jane Adventures and K9. He also voiced the Nucleus in the Doctor Who television story The Invisible Enemy, played Dugeen in the Doctor Who television story The Power of Kroll, voiced the Dalek battle computer in the Doctor Who television story Remembrance of the Daleks and played Anthony in the Reeltime Pictures spin-off video Downtime. He also provided the uncredited voice of the computer in the Doctor Who television story The Sun Makers. (DWM 306) On audio, he reprised K9 for two Adventures in a Pocket Universe audio dramas by BBV Productions and in Big Finish Productions' Doctor Who and Gallifrey audio series and reprised the Nucleus in the Big Finish Doctor Who audio story Revenge of the Swarm. He also voiced Chief Mover in Magic Bullet Productions' Storm Mine, Machina in The Paradox Planet and Legacy of Death and the fifth incarnation of Drax in The Trouble with Drax.
Tom Baker detested the character of K9, but got on very well with Leeson. A favourite joke during filming breaks was for Baker to work out crossword puzzles, asking the K9 prop the clues with Leeson giving the answers in character. The short sketch Merry Christmas Doctor Who involved K9 telling the Fourth Doctor and Romana I that for Christmas he would like "enough ball bearings to last the series through and fewer sill irons on the floors of the planets we visit," as well as the robot dog downing some gin before beginning a drunken rendition of "We Wish You a Merry Christmas". When Leeson appeared on Tom Baker's edition of This Is Your Life in 2000, Leeson entered on all fours.
Leeson himself became frustrated with the character. He left the programme after the conclusion of the 16th season in 1978. David Brierley took over for four stories (one of which was never completed due to a BBC strike) the following year. When John Nathan-Turner took over as series producer for season 18 in 1980, he tempted Leeson back, with the promise the character would be "killed off" that year. Although the character was, eventually, not killed off, merely written out, Leeson agreed to return, and portrayed all of K9's subsequent return appearances on Doctor Who and its spin-off shows.
Other credits include appearances in Blake's 7 (season B episode "Gambit"), Jigsaw in which he played the giant Biggum, who was so tall only his feet were seen and his voice heard, Sorry!, 'Allo 'Allo! and Bungle in the children's television series Rainbow. Leeson is also the voice behind the Space Gypsy characters on Children's UK hospital radio.