Isn't that kind of strange, though?
For example, you finish the master thesis with a mentor and both he and you would like to continue with the mentorship in a PhD. As he was your mentor, he will write a recommendation, however, as he would be your future mentor, the recommendation is most relevant to him as well.
Or does the department/University need additional persuasion besides the wish of their professor to take you as their student? The research topic approval maybe?
Basically, is it possible you are offered (informally) a PhD place at the end of the MPhil or do you have to start applying in November (at the beginning of the MPhil)?