Regularly get 50+mpg on the way home from work - 40 miles from our D240, best so far is 55.7mpg for the trip. It's the 5.0 / 5.0 / 5.0 on the driving style that I would be suspect of.
OK 4.5 / 4.9 / 4.8 would be good for me, then again 1.2 / 2.0 / 2.8 is possible as is 26mpg for the same run.
Lend the car to a teenager for an hour and then check the scores, if they are above 3.5 the car is lying.
My best trip is 57.5 MPG in current D180 manual. Average over the 40k miles is 41.79 using fuel litres added and miles travelled and 44.1 if you sum the trip computer MPG over the same period (just a few percent optimistic).
You figure is credible, especially if you've had a mostly downhill trip