No idea, but I would say that, bar getting an estate agent to value it, they could use the property value index used for the property tax as a rough guide. I had to give them my own estimate when filling out the bankruptcy forms and used this guide.
I've vacated it but the OA has not put it up for sale yet. Afaik He has 3 years to sit on it for the market to continue to rise, I don't think it goes straight back to bank. Property is in over 150k negative equity. I don't think either party would rush to sell.