I had my interview at the alum's office in central london. It lasted about 1 and 1/2 hours and it was fairly informal. The interviewer must have memorised my application - she didn't even need to look at it during the interview, which meant that her eyes were trained on me the ENTIRE time. She grilled me on all my essays, so it would be a good idea to go over yours again before the interview.
Oddly enough the presentation was the easiest part of the interview. She picked a topic from a list she had, then left the room for 5 mins to let me prepare. Afterwards she said she was mainly looking for evidence that I could structure my response properly. In hindsight, I'd say the best approach is to break yours down into 3 sections - Intro, Main body and Summary/Conclusion. But whatever you do, pick a side and defend it. It's very much like the AWA in the GMAT.
Good luck with your interview!