Simon...
I agree with Tim on shrouding the fans..it really makes them much more efficient, and stops a lot of bypass air that can sneak in from behind the radiator. Also, something I have found on electric fan setups is that if your fans come on at speed they tend to slow the air flow, actually regulating it to whatever the fan can pass to a point. Most OEM setups shut off the fans at 35 to 40 MPH regardless of engine temp., as the flow at driving speed is always higher.
Phil