Just an update...

Having temporarily mounted the duct up the car I was concerned about the amount of air actually feeding into the duct. I should have thought about this earlier but due to the shape of the valance and the position of the duct almost on the corner, the airflow tends to move across the face of the duct rather than into it. So I decided to reshape the duct to avoid this cure this. I think I prefer the smother, blended look of the original but if its not doing the job then what's the point? Anyway here's a couple of pic of the roughly reshaped duct - maybe the more aggressive style might grow on me.

Dan, let me know if this is still acceptable for you.

Simon.