:Oops: I'm an idiot!! please ignore my last post !!
It's been off the car long enough for me to forget how i put it on. It does of course tuck under the top lip of the grill. I took it off because my new grill doesn't have a top lip. So it doesn't, in fact, block the airflow.
However,on the stock bonnet at least, there is a rubber seal that sits down on the bar that runs across in front of the radiator..thus preventing leakage over the top.
I stand by my remark re the gaps at the side being more important. The cooler plate does nothing to address the losses here that are substantially more than any loss over the top.
Now, any ideas why the car ran 2 or 3 degrees hotter when it was on.