I looked it up when the question first came up. You will find some Youtube advice both on the advanced ABRSM exam you are doing - I looked up "ABRSM + viva voce" and this hour-long thing came up.
You can look on the Diplomas section in the ABRSM Forums. I found a reference to a Webinar by a piano teacher who teaches at this level.https://m.youtube.com/watch?v=262vC71GhnQ&feature=youtu.beAt about 4 mins he starts talking about the Dip ABRSM in general and around 20 mins he goes into the Viva Voce expectations in detail.