The most important point to remember is that you are responsible for the smooth running of your laboratories. Your demonstrators are working for you, thus you should permit them to get on with simple things they are good at, like marking and helping students with technical and academic problems. You should act as a supervisor - dealing with all extraordinary cases and problems either immediately or by seeking assistance as is required. You should not see yourself as another, possibly ``glorified'' demonstrator. It is thus likely that you will do little of the marking (although if you wrote the marking schemes it is obviously a good idea for you to do some marking to make sure they work).
Avoid getting bogged down in marking or helping the students: many of them will think you are the only person that can help them. Feel free to pass on their requests for help to one of the demonstrators, so that you have enough time to actively supervise. Ensure your demonstrators are actively working: encourage them to wander around the room looking for people who need them, rather than standing in a corner chatting.
Make sure your demonstrators know that any irregularities and absence or deadline-missing excuses should be discussed with you. Students often fail to appreciate that they should report such things to the supervisor rather than mention them in passing to a demonstrator. When such are reported to you, please make sure the proper record is made in the ARCADE sheets - the process is simple enough, but causes considerable hassle to the students if it is not adhered to. If you are relatively inexperienced in ARCADE, and are not sure what to record in a certain situation, you might seek advice from one of your more experienced demonstrators, from the team of data-entry demonstrators in the Student Resource Centre, or from John Latham (email: jtl@cs.man.ac.uk) or Alan Williams (email: alaw@cs.man.ac.uk) directly.