Ecto-Space

Ecto-Space was the name given to a pocket universe by the Mistress after she and a version of K9 and his Mistress began travelling there, fighting slavery and oppression. (AUDIO: The Choice, The Search)

Ecto-Space's physical laws, including quantum mechanics, were somewhat different from the "prime" universe's, which meant the two travellers struggled to construct a space-time ship that would work correctly. However, it did have a Space-Time Vortex similar to the one they were already familiar with. (AUDIO: The Choice)

At some point, humans also found their way into Ecto-Space. They founded multiple colony worlds, including Sherwood.

At least five hundred years after the first human incursions, Cyberons also made their way into Ecto-Space, attempting an invasion of the entirety of Ecto-Space, but they were ultimately "beaten". (PROSE: The Choice)

An unknown force within Ecto-Space once created a device capable of creating fractures between Ecto-Space and P.R.O.B.E.'s home universe. The device was lost by its original creators and "drifted" into said main universe, where it was acquired by Control, of the CIA's branch dealing with extraterrestrial and supernatural matters. Control's people experimented clumsily with the device, causing rifts to begin appearing between the two universes. An entity from Ecto-Space tried to "cauterise the wounds" between the two universes using a mysterious pink slime, but said slime's interaction with 21st century humans itself had undesirable consequences. Ultimately, the CIA called P.R.O.B.E. for help in late November 2020, and Giles deactivated the device under Maxie's remote guidance, ending the brunt of the crisis. (HOMEVID: Goo!)

However, as late as March 2021, P.R.O.B.E. was still dealing with fallback from the incident, as an extradimensional entity which had leaked through into the universe from Ecto-Space through the pink slime, and then merged with a film reel of a famous German horror film, began to manifest in the physical world as a version of the movie's main antagonist. (HOMEVID: Living Fiction)

Behind the scenes
Ecto-Space, as portrayed in BBV Productions' Adventures in a Pocket Universe series, was implied to be E-Space, and its characters the Mistress and K9 were implied to be Romana II and K9 Mark II following the events of Warriors' Gate.

Though BBV secured permission to use K9 from his creators and rights-holders Bob Baker and Dave Martin, they did not obtain the BBC's licence to use Romana, E-Space, and other elements of the Doctor Who universe. No explicit connections between those BBC-owned properties and BBV's counterparts to them could therefore legally be drawn.