That's a specific convenant put into your land title register, Imd, it isn't very common. Jake is broadly right. Local Council can take action but will charge you an assessment fee. There are guidelines produced now that enable Local Council to assess the nuisance caused by the hedge. They don't depend only on the height - they depend on the combination of the height, the distance from the boundary, the length of the boundary impacted by hedge, the orientation of the hedge (one on the S side takes more light off).