We have finished the watercourse between the top and bottom ponds. First thing today it looked like the photo below, and then it poured with rain so we collected quite a bit of water from the guttering on the greenhouse.
With a bit of trial and error we ended up with a meandering stream that starts with the overflow to the top pond, then the water dropped into a wide pot, down a concrete gutter to another pot, through a pipe into what Darren calls the dalek, and finally down another gutter into the bottom pond.
Not sure yet how attractive it is, but I think that it will be fun. I think that the fish will like the trickling waterfall, just as long as they don't try to slide down it.