The process is very casual and informal. You will be called up from reception to the trading floor and taken into a meeting room. There will be two interviewers usually a Senior exec/manager, they will most likely be in the team you are joining. So i would advise finding out who they are and what accounts they work on. You will get a maths (just ensure you know how to work out percentages) and english test (just basic english skills to ensure you can write an email).