I already have a masters degree, and I too have a PhD interview coming up; which I'm dreading to be honest!
I just want to start the PhD without having to go through all this crap as I hate interviews.
I would imagine that during the presentation phase they may test your knowledge on the PhD topic. However, they will not be looking for you to know everything at this stage, as it would defeat the purpose of doing the PhD in the first place. Furthermore, the MRes will get you up to speed on current issues in the field that you're doing your PhD in; so it's definitely a good thing to do!