One more thing: once the syphon starts, it tends to "self-perpetuate". As hot flows out the panel and into the header tank, that tends to "push" water out the bottom of the header tank, which in turn "pushes" water into the botom of your panel.
I've seen systems with high differential temperatures (wood fired boiler) with 1" pipes (not withstanding nearly 100 feet of pipe between the tank and the boiler) making flow rates of over 60 litres/minute without any active pump!