Thanks for the tips.
I'll try a larger fire first, and can add more ventilation if required. All of the build guides I referenced advocated either a 4 x 3/4" or 3 x 1" configuration.
I lifted the deflector out initially when it seemed that it might be stifling the fire. Using one seems to be a bit of a religious argument, depending on whether you like meat juices in the coals or not.
The ventilation in the lid wouldn't be to little or too great would it?