McMillen

You may be looking for DI McMillan.

McMillen was a woman in the 42nd century who possessed a book with alien writing in it. It talked about the Galis Expedition. She found out that all they had to go on was local myths and that there was a black hole called K37 Gem 5 orbited by a planet. She asked Captain Walker if he could bring back the power source. (WC: Tardisode 8)

Behind the scenes

DWMSE 14 gives Ri Richards' role as "McMillen", several times in its overview, but "McMillan" in the cast list, presumably contrary to the shooting script.