Doing a postgraduate degree is a full-time job in itself. You might be able to earn a little cash by working during your spare time, but the university has strict rules regarding the employment of graduate students. These are stated in the Memorandum to Graduate Students, which is issued by BoGS and is available for consultation at the Graduate Union. Many funding bodies also impose similar conditions. If you wish to work you should ask your Senior Tutor or Graduate Tutor for permission.
Graduate students are routinely employed by the university teaching undergrads, invigilating exams, working in libraries, or taking notes for students who cannot do so themselves. Casual work in pubs and commerce is also generally pretty easy to find.