From TARDIS Index File, the free Doctor Who reference.

Adventures in a Pocket Universe were two audio dramas released by BBV Productions under their Audio Adventures in time and Space banner.

These two audio dramas The Choice and The Search featured John Leeson as K-9 and Lalla Ward as 'The Mistress' (thought heavily implied it is Romana II), Lalla Ward's character was only ever known or referred to as 'The Mistress' (which was how K-9 originally referred to her) as the character of Romana was governed by the BBC license.