Dear Sue, I am typing the following text from "The Research Student Guide to Success" by Pat Cryer (viva section). It is an Open University publication, so it can be trusted: "Many students prefer not to have their supervisors with them at the oral examination because it can be inhibiting to explain their works in front of someone who knows it so throughly already. Supervisors can, however, be present in certain circumstances, depending on institutional regulations. So you should think about whether there are good reasons for this to happen in your case, and then discuss possibilities wih your supervisor."