Well I finally passed my PhD viva with minor corrections. I honestly don't know how, as I think I only answered about 25 % of the questions that I was asked ! I can only assume that it was the strength of what was in my thesis that got me through. I definitely got brain freeze on the questions and was thrown by some of the questions I was not expecting. From my experience make sure you know everything about the figures in your thesis as 90% of my questions concerned them. Anyway just glad its over.