My 150DSG has just returned 40.7 MPG over a trip of 519 miles (filled up brim to brim - 58litres - 12.76 gallons) - MFD was showing 41.3 so pretty accurate. Mainly motorway and good A roads, driving at speed limits, with a few traffic jams. There was also a bit of Eberspacher heating included but, running for just a few hours, this would have been no more than a litre so I have not bothered adjusting for it. NB. This was using BP Premium diesel as when I pulled up at the pumps last week they were out of the normal stuff - presumably this will have slightly improved the MPG.
I'm pretty sure that when I first got the van I was getting better MPG than this (trip to Bala last year, slower speeds but towing a boat) I got nearly 50 on the way. Consequently I suspect the "software update" applied when my van went in late last year may have had a detrimental impact on the MPG??