The past papers for this subject has been grossly inaccurate as the lecturers change every session and you can't really expect them to ask you to design the same system each year. As long as you know your OO theory, class diagrams, DFDs, you should be fine. The exam last year was incredibly long because they wanted a full DFD (all levels) which took ages to complete. My diagram has arrows everywhere (crossing over pages, etc.) because there just isn't enough time to arrange it properly. It might be better this year.