its difficult to compare as we have different cars, mine is a 140 common rail remapped to 180. i get around 500 with a mix of driving which works out to an average of 35mpg which isnt bad considering my right foot is constantly planted to the floor. my trip counter + remaining is currently saying i'll get 650 which would be 46mpg, however i've done a 140m round trip on the motorway (85+) and back with no local driving since i last filled.
oh, and i use biodiesel which is said to reduce MPG by around 10%
Either way, 400 sounds low, if your tank is the same size as mine? 65l. that'd be an average of around 28mpg!