I watched that "It's not easy because I am making the programme for the BBC being Green" programme this evening. Work was started on the waterwheel.
A few of things struck me about the design. If I had the money to waste I'd have made the source come from a sump of some sort -however small. And have the delivery on a deeper section; perhaps a vee chute with the sides at an angle of 60 degrees. This would allow for a faster flow and the maximum economy of trough material.
On the wheel, the cups could have been designed to hold as much of the water as possible at the rim. And incorporate a Teflon coating on -once again, vee sections; more of them and centring the water on the wheel to diminish flexing. Thus the wheel could be made narrower and slightly lighter. In fact I think the wheel could have been made without rims. There is very little weight on the amount of water they had at their disposal after all.
Perhaps incorporating a vee section holding the spokes apart/together instead of a rim, making them the cup sides? I chose 60 degree vee shapes because they are the simplest section and the angle 60 degrees because it is the best ratio of sides to volume envelope.
Pity I'll never get a chance to find out.