If the dealer was quoting 48mpg I would suggest he is being less than honest as you would never expect to get that in the real world.
My previous 177ps was supposed to deliver 47mpg IIRC and I averaged 36.5mpg over 16,000 miles.
The 190ps is supposed to give similar mpg to the 177ps but
a
you cannot rely on the DIS and
b
I have always found the first tank full to give very poor mpg as the engine is very tight. You need to do a lot more than 250 miles before you can get a feel for consumption. Having said that, I have done over 520 miles on the first tank, but I have been on holiday and not doing my normal commute.
Let us know how you get on.