Its worth remembering that there are loads of different factors when it comes to MPG.
Fuel: Different fuels DO give better/worse MPG. Additives can
help if the fuel is decent to start with.
Tyres: Differebt size and width of tyres have big effects on MPG. the wider or bigger the tyre, the more surface area on the ground and the more friction for the car to overcome.
Weather: The weather can make a difference. Cars run better on cold damp mornings than hot dry ones.
Driving style:If you hammer the car, it will drink more fuel. If you keep the car in the peak torque range it will drink less. Sticking to top gear in a 40mph zone isn't always the best way to increase MPG.
Distance: If your driving a couple of miles to work and back your MPG will suffer as the car won't warm up very quickly.A longer journey will see better MPG overall.
Age of car: The thing most people forget when buying a diesel is the condition of the engine. It may be brand new, but that means it's tight and still has some loosening off to do before MPG increases. There are lots of figures being thrown around on what milage you car should have loosened up by. I think 20k is somewhere near the start of it.