Hey Kev, someone might correct me on this but I think you only get the one shot- the outcome of the viva will be pass, pass with minor corrections, pass with major corrections, revise and resubmit, then an outright fail or a decision to award an MPhil. Obviously if you get corrections (and hardly anyone passes without any!) then you're fine, a revise and resubmit is basically a second chance but I think you can only do this once (sometimes this requires a secons viva but not always), and then that's it. So best to try and get it right straight away! I might be wrong, but I think that's how it goes- if in doubt, check with your uni! Best, KB