Trivia: "Shada" is famous amongst Doctor Who fans for being "the show that never was." There were to have been six episodes of the story, scheduled for transmission beginning 17 January 1980. Location filming did take place in and around Cambridge in September/October 1979. However, studio recording was hit by a strike by BBC Technicians in October 1979. When the strike was settled, the BBC studios were all fully booked for other programs, so "Shada" could not be completed in time for its scheduled showing, and so it was never completed. (There was an attempt to make "Shada" the first story of season 18, but by that time, several key people, including writer Douglas Adams, had left, and the story was formally abandoned and cancelled in June 1980).
Trivia: Plot elements from this episode and "City of Death" (the Cambridge don whose flat is a time machine, and Scaroth's mucking with history to prevent the accident that stranded him on Earth) formed the basis of Douglas Adams' novel Dirk Gently's Holistic Detective Agency.
Answer: TARDISes are generally available for properly authorised use on Gallifrey; they're not usually assigned to a particular Timelord on a long-term basis. The Doctor stole his when he left his homeworld.
Tailkinker ★