The Monk is Missy's new companion Volume 3 and the good news is its still Rufus Hound playing him despite his recent Twitter scandal that got him kicked off Dancing on Ice
https://www.bigfinish.com/news/v/missy-s-meddling-once-again
So in the article that announced the return of David Troughton as King Peladon in the upcoming Peladon box set Big Finish teased the inclusion of "Various time travellers also turn up across these tales – and some of them have very familiar faces..." who do you think they will be? My guess is...