Also, having read some of the comments above, there seems to be a bit of confusion as to whether the dyno gives bhp at the wheel or fly.

A lot of rolling roads now do it at the fly and they do it by measuring the BHP at the wheels as they go up through the gears and then the machine continues reading when they completely let off the throttle. By measuring the resistance of the car across the rev range as it is slowing down, they can calculate a fairly good estimate of what power is being lost by all this resistance which they then apply to the wheel BHP to give you a fly BHP at the engine.