If you drop the speed 5km/h to 100 I believe your economy will improve disproportionately. When I did my last big trip in the 300tdi at 90kmh, I had to stop every now and then to let fuel out!:D
Also I have heard that aircon can improve economy on modern cars because the windows are all closed and thus aerodynamics is improved. I can't see this being an issue with Defenders though
