For applicants that have been invited to an interview, congratulations!
The impromptu presentation can be a cause for concern for some of you since it is going to be a "surprise". I must say you need not be anxious, because I do not think they are looking for the content within your presentation. Well it has to at least be logical! I do not know if they keep the topics consistent, but I can tell you this. If you can tackle the case study given to you (and you probably can, since it's a required part of the interview), you will have no problems with the impromptu presentation!
I recommended Case in point in an earlier post if you really want to feel prepared for this section. It will help by giving a structure for tackling the presentation topic. With that said though, the book felt like an over kill to me. Again, I do not think the content matter as much as how you deliver your presentation.
I leave you with three thoughts:
1. Be confident
2. Be logical
3. Be concise