How to value 1/3 share of a property?

My client owns 1/3 of a residential property, his father owns the other 2/3.  He wishes to gift his1/3 share to his father.  How would this share be valued for CGT? i.e. what discount would be sensible for a minority holding of a property?

He hasn't lived in the property for many years so it is not covered by PPR.  His father does however live in the property.  His father is also in his late 70s.