First of all, congratulations on getting an interview!
I was in the same position as you when I had the interview for my PhD - very short notice and I was very busy with other things.
Whilst they've told you it's a 'relatively informal discussion' (and it may well be), I would prepare for a proper interview. Just read up on the background of the university, department and the potential supervisor's research, as well as the background to your research area. Think about the obvious questions they may ask you: describe yourself in a few words/strengths and weaknesses/difficult times and how you overcame them etc.
I think I also just spent a day or two trying to understand 5 to 10 key papers on my topic and these came in very useful when they asked me specific questions about my research topic.
Smile, be confident, positive and enthusiastic and you'll be fine!
Good luck!