At the end of The King's Demons the Doctor admits he gets her to stay with the TARDIS crew by basic button pushing. In Frontios she gets so annoyed by the Doctor's "she's my android PA" ruse ("I got her cheap because the walk's not quite right. And then there's the accent...") that she nearly gives the game away.

And anyone who dresses like she does in seasons twenty and twenty-one while surely knowing she's going to do lots of running around, usually on quarry-type planets, is asking to have either her sanity or her dress-sense questioned.