Hello,
I don't know anything about your area but I would suggest that you could talk briefly about the context (i.e. the extent of the problem, cite some existing literature if there is any) to show that you have already done some exploratory reading in the area, and then highlight some interesting questions and issues that have emerged for you. I don't think you need to put forward a whole proposal about how you would go about researching the topic (unless they have implied this in the information pack) but be prepared with some ideas so that you can answer questions about it. Obviously, if everyone else in the research centre/department is qualitative or quantitative they are probably looking for someone with a similar methodological approach, unless they specified otherwise in the advertisement.
At my interview I had to give an "informal presentation" which totally baffled me but in the end I simply talked for a few minutes from notes and then discussed the ideas with the panel members. They asked me a lot of non-research related questions too, like why I wanted to do a PhD, whether I knew any other doctoral students and understood what was involved, and so on.
Hope this helps. Good luck!