Straxus' TARDIS

Straxus owned his own TARDIS. Sandun was a technician that helped him pilot it.

The Sixth Doctor physically expanded the dimensional stabilisation field of Straxus' TARDIS to the entire pocket dimension that the Decayed Master had created, to allow his fourth, fifth, seventh and eighth incarnations to fully materialise their own TARDISes in the dimension itself. (AUDIO: The Light at the End)

He used it to save the Eighth Doctor and Molly O'Sullivan from a group of Daleks. It was attacked by the Daleks and he escaped back to Gallifrey. Straxus later travelled to Srangor to kill himself and prevent his future incarnation; Kotris, from ever being created, as regeneration was impossible there. (AUDIO: Fugitives)

After many attempts to find the Doctor, there was a power failure within the TARDIS. After this failure was corrected, they managed to find the Doctor. (AUDIO: Tangled Web)

Sandun sacrificed himself to make sure that the Doctor did Straxus' bidding, and show him that the TARDIS was time rammed with the Doctor's TARDIS. (AUDIO: X and the Daleks)