A few more notes, im having a cook as we speak. The only time I could really get a decent temperature was by leaving the burner door open, so as a result I have drilled two more intakes. It has definitely improved things.
As a test I put a hood thermometer on the grill and noticed this was sitting nicely at 300 while the hood was only showing 250. My exhausts meet the rear in a tear drop shape, is it possible that my exhaust design is too efficient and therefore letting out too much heat?