Excelis Dawns

In 2002, Excelis Dawns was released by Big Finish Productions.

Cover blurb
That terrible old reprobate and transdimensional adventuress Iris Wildthyme has gone and locked herself up in a nunnery on the savage world of Artaris. Here she is discovered by the Doctor and the reasonably brutal warlord Grayvorn. Together - with a peculiar nun they pick up en route - they must travel the forests and swamps of this ghastly world in Iris's double decker bus in order to tussle - to the death - with a horde of flesh-eating zombies for a mysterious and holy relic of unfathomable value and questionable pedigree.

But what is the Mother Superior's part in all this?

This story takes place during Frontios.