*Assuming the pretty drawing is as fitted, I wouldn't expect it to work.
*Also I don't see this schematic on systemlinks website which shows the cold feed/vent arrangement connected on the gravity circuit
The pretty drawing was drawn by Systemlink
*the bottom coil should be used for the gravity, or the middle coil at a pinch
I should clarify that the top and middle coils are intertwined in the cylinder, not at different levels as shown by the drawing. Attached drawing, number 5 1800x450
*there's no provisions to vent the primary circuit of air.
The vent is on the primary circuit. The Systemzone box has an auto air vent.
* the cold feed and vent must be on the gravity circuit.
Good point. The water cannot expand to begin circulating.
*there should be cross flow across the boiler.
There is cross flow. This drawing omits that detail.
*I'm surprised no one has questioned the cold feed which is impeded with valves and a circulator!
Nope - gravity circuit has no valves, circulators, pumps or anything on it. As shown in the pretty drawing.