You can easily look it up on the land registry if you know their address. It's all in Ant's name, however as you suggested they could have a separate legal contract which indicates she owns a % of the house? I have done that with my partner, I purchased the house, it's all in my name on the land registry, but we have a legally binding document that he owns a % of the house.
I do find it weird as it would have been perfect to put the house in Zanna's name as I assume she hasn't got a property, so if he'd used her name they would have saved a load of £££ on stamp duty, whereas Ant has owned before, so not his first residence. Hmmm either doesn't trust to put it all in her name or doesn't care about saving a fortune in stamp duty?