Hi there, I have a 3.0 DCi Auto. I don't know how many miles yours has covered or if its manual or auto, but I wouldn't say that these figures are worryingly low (they are quite close to mine). It has to be borne in mind that these cars are very heavy, so a smaller, less powerful engine (albeit more efficient - 4 cylinders vs 6 cylinders), has to work harder to haul the car around. Plus, they aren't the most aerodynamic shapes, having to punch quite a sizeable hole in the air. (The reason I state this is in the hope that your fuel economy expectations haven't been raised due to 'official figures' often being quoted).
I found using a diesel fuel additive helped to increase the economy, and indeed averaged 43mpg for a 700-odd mile round trip to Wales. This included the Wife & baby, plus luggage.
& filters will make an improvement, as will ensuring your tyres are at the correct pressures, and trying to drive with economy in mind (i.e. looking ahead, changing up early, and so on). Fuel economy also drops at this time of year due to the engine running rich for a greater duration, plus all the extra ancillaries being used take their toll.
Enjoy your car, you are in a very exclusive motor.