Ok, just tried out my temporary cardboard grille block. For anybody who's interested, I've put all the results on the MG-Rover forum here:
The Super Efficient 1.8 project - Page 4 - MG-Rover.org Forums
I'll definately stick to the engine in neutral but idling technique for the moment, as I want to see how I can reduce fuel consumption on idle. The hot air intake certainly seems to contribute to this very effectively.