I also work as an IT contractor in a large investment bank in the city. Have been contracting for the last 8 years. The whole dept. are contractors bar a couple of managers. All "staff" have set shifts we are required to cover. The shifts are advertised as such and no we cannot decide to just leave when we want to and come in when we want to. think you'll find they'll get rid of you in a heartbeat and get someone else in.