Found this ref in my CATS ATPL study guide hope it helps clarify:
Key point is that there is s published minimum alt/height for visual circling
Descent below MDA/H should not be made until:
• visual reference has been established and can be maintained;
• the pilot has the landing threshold in sight; and
• the required obstacle clearance can be maintained and the aircraft is in a position to carry out a landing.A circling approach is a visual flight manoeuvre. Each circling situation is different because of variables such as runway layout, final approach track, wind velocity and meteorological conditions. Therefore, there can be no single procedure designed that will cater for conducting a circling approach in every situation. After initial visual contact, the basic assumption is that the runway environment (i.e., the runway threshold or approach lighting aids or other markings identifiable with the runway) should be kept in sight while at MDA/H for circling.If visual reference is lost while circling to land from an instrument approach, the missed approach specified for that particular procedure must be followed. It is expected that the pilot will make an initial climbing turn toward the landing runway and overhead the aerodrome where he will establish the aircraft climbing on the missed approach track.As the circling manoeuvre may be accomplished in more than one direction, different patterns will be required to establish the aircraft on the prescribed missed approach course depending on its position at the time visual reference is lost.