Next universe but one

The First Doctor told Ian Chesterton that they were "already in the next universe but one" when the TARDIS landed on Skaro, thus making the human's preoccupations about his planet irrelevant according to the Doctor. (PROSE: Doctor Who in an Exciting Adventure with the Daleks)

Behind the scenes
According to AUDIO: Quinnis, which picked up the thread of the mention, in The Edge of Destruction, of the First Doctor and Susan having travelled to the fourth universe, N-Space was the third universe. Logically, if the Doctor's statement in Doctor Who in an Exciting Adventure with the Daleks is to be understood within the same framework, this would make the "next universe but one" the fifth universe.

Genesis of Evil also posited that Skaro did not quite exist within the Doctor's universe, but placed it in hyperspace rather than in "the next universe but one".