This is how I think it works!
As the water rises in the reservoir it flows up through the inner ring of funnels and down the overflow pipe. A further increase in water level will bring the outer ring of funnels into operation. Eventually as water level rises even more the outer rim (weir) will be overtopped. I'm not sure what the benefit of this design is over a conventional bellmouth but it would perhaps start overflowing more gradually than a simple wier.
John Simpson (with help from Paul Bingham)