I have only had my transfer viva, but I found that when it came around I had been working on fine details of coding and analysis the few months before so I wasn't really looking at the big picture. Remember that your examiners will know less than you about your project, so will not be asinterested as you in the fine details.
I would make an effort to take a step back, try to be able to sum up what results you have in a few sentences (Hard I know!) and have a good idea in your head as to why you are using the particular techniques you used, what you are doing that is different to what others have done before.