HSPS written work and supplementary statement

Hey, I'm applying for HSPS at Cambridge and I was wondering if anyone knows what kind of work to submit when they ask for written work. I know they want something written in school and marked by a teacher but apart from that I'm not too sure. I was also hoping someone had advice for the statement on the second form they make you do. My UCAS statement will focus on sociology, and I want to study that track but I don't have the most knowledge on the other three subjects. Any ideas for the best approach? Thanks