Continuity mistake: In this episode part of the TARDIS set was erected incorrectly, resulting in some of the roundels emerging from the wall rather than being indented into it. This is very visible in all the TARDIS 'console room' shots.
Other mistake: In episode 3, watch the scene where the co-pilot is shooting; you can see his pants rip straight down the seam at the back - which unfortunately happens to be the part of him facing the camera at the time. He then wanders around for most of the rest of the episode with the tear in his pants plainly visible.
Continuity mistake: In episode 4, approximately 5 minutes from the end of the story, Soldeed is shot after he pulls the lever that sets the complex on overload, and then crumples to the ground dead beneath the lever. Then, after the Doctor and everyone else get out of the complex, we see a selection of shots taken inside the complex - one of which is the room with the lever that Soldeed pulled, except he's no longer there...
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 ★