I had the same worries with a 159 1.9 jtdm which only returned 39mpg on motorway runs from Bristol to N. Yorks but that has steadily improved, so that after 40k miles, I now get about 46mpg on the same run, which is near the quoted 47. I have checked the computer with fill-up to fill-up and it is very accurate.
The biggest improvements was after first service when I had fully synthetic oil used, but it has continued to loosen up - or maybe I am getting older and slower! The biggest surprise for fuel was a non-stop run when my wife was taken ill, from the south of France to Calais on a Sunday on totally empty French motorways at a steady 80, and it gave a surprising 48 mpg and did it easily on a tankful.
Suggest you hang in there, try to be a smooth driver and use the recommended fully synthetic oil - the main agents only use semi-synthetic because the other is so dear - but I buy mine in France, or from Ebay recently, and take it for the service at a local independant garage.