Hi,
It will also depend on what station is on the radio, I have found the BBC ones tend to be better, especially BBC 3 at helping MPG figures to climb, usually because I'm asleep in a lay-by after about 10 mins
I have a manual TD5, 04 MY, and I regulary get 30+ mpg, (I even manage approx 26-28 when towing a caravan) obviously it does make a difference when it is cold, but to be honest I can count on one hand the amount of less than 10 mile trips I do . . . so maybe my figures are slightly better than the 'avarage driver'
Also I appreciate a manual will be better than an auto, but a well serviced Disco should manage 26+ in auto and 28 - 30+ in manual
Servicing will to a degree make a difference, but not to the improvement that Partmaster is hoping for, that suggests it his may be over fueling, so get this checked out
Any LR garage will do this check for a small fee (unless you go to a main stealer !!)
DIY checks can be the airfilter (try fitting a K&N one !!) and check the whole air intake route for dirt/leaks etc from the filter all the way through the intercooler to the turbo etc.
In particular make sure the airflow meter is clean and functioning as this feeds info to the ECU for correct fuelling, if you still have the dreaded EGR, either blank it off, or clean it throughly and when you're happy with that lot, fill her to the brim, drive a steady (but not old codger speed) route of at least 50+ miles to ensure the engine is fully hot (check the temp guage says 'normal' (thermostat check), and then brim again and calculate as follows (sorry if you know all this, but others may not)
Miles driven
Qty of fuel in litres = miles per litre X
Then divide this value by 0.22 to give the mpg
Town driving makes a big difference as you would expect, if you have an auto, and floor it and use 'kick down' a lot you should expect poor mpg, but if your that worried about it, I suggest you get a smart car, Toyota Hybrid or a skate board, or you could always fit a sail to the roof of your Landy and catch all this crap windy British weather -
Afterall - It is almost 3 tonnes of metal that is as aero dynamic as a breeze block