Here's a picture of the filter housing. I've placed another slate in front of the waterfall because the noise was a bit loud. So it's now a weir, not a waterfall.
Gah. As much as I like to see birds in my garden, I really dislike the mess the pigeons make!
I fixed two handles to the 2' x 2' slate as it weighs a ton and kept trapping my fingers.
Here's the filter. As you can see, the water discharges onto a shelf. I fixed some plastic pipe at the back of the shelf to stop water going back into the housing. The water used to come out too fast and hit the bricks in front of the the outlet, so I placed a slate in the outlet to guide the water into the opening.