Have you read Getting a PhD in Law by Morris and Murphy? That has a chapter on the viva, I believe, and can also help you to ask some examiner-style abstract questions about your PhD (about methodology, especially, which is tricky with law). Otherwise the usual viva questions apply: what does your argument add to what's out there already in your field? What's the practical impact, if any, of your argument? Can you make a list of likely counter-arguments to your thesis and come up with how you would refute them? If you have a time, it might be worth skimming some recent work by your examiners. That will usually give you some insight into their methodological angle and allow you to predict the kind of thing they might ask. Good luck!