Students that are funded by the school are expected to be available to dedicate up to six hours per week (during term time) of their time to working on one of the modules the school teaches. This might be a programming module, or a theoretical module, or something else. If you are a TA on the 4-year option, the you will be required to work 12 hours per week.
What Demonstrating Is
The idea is that you do up to six hours work marking, or preparing for teaching, or demonstrating in a lab, for the course of your choice. It goes without saying that you should be skilled in the subject you demonstrate for. You're not allowed to work more than 6 hours per week while you're a research student here (unless you're a TA), but luckily the pay is good and demonstrating can significantly boost your CV. You are also no allowed to work more than 180 hours of demonstrating each year in total.
For more information on demonstrating, see http://www.cs.bham.ac.uk/internal/research_students/demonstrating.php
Really? How Good is the Pay?
Well since you asked, pay falls into two categories. There's the postgraduate demonstrator rate, and the postgraduate small group teaching (or SGT) rate.
You'll receive the demonstrator rate for all work that you do that doesn't involve teaching of small groups. As of 2012/2013, it is £14.50 per hour.
For small group teaching, where you teach a group of around 20 students, you will receive the much nicer £29.00 per hour (as of 2012/2013). You claim for this every month (or as often as you prefer), but will be taxed unless you've filled in the Demonstrating Forms. Be sure to submit a form at least once a month and at least a month before the end of the tax year to ensure you are not loaded with a hefty NI/Tax bill. If you get charged too much NI it is very difficult to get it back (and impossible after the end of the tax year!). If you get charged too much tax you may have to wait until the next April to claim it back and the Inland Revenue aren't anywhere near as fast refunding you as they are at billing you.
Sounds Great. Where Do I Sign Up?
It's quite easy, just get in touch any time with the TA/demonstrator tutor - currently Peter Tino e-mail
Generally you have more chance of getting a demonstrator job at the start of the academic year. However, in the past few years there always have been unfilled vacancies, so if interested at all, please do get in touch!