I would suggest that you figure out what MPG you are getting, which will make comparisons easier and more accurate, and give you a better idea the scale of the problem, if there is one.
- Fill the tank.
- Zero the trip.
- Next time you fill up, fill the tank to the top again, and note the quantity of fuel used.
- Note the miles travelled.
- This will tell you your mpg.
- Post you mpg, here and, the C30 section would probably also yield some specific results.
Believe you have around a 12 gallon tank?
1/2 tank, 6 gallons, 300 miles = 50mpg.
3/4 tank, 9 gallons, 400 miles = 44mpg.
Honest John website suggests an average 51.1, so your half talk appears in the right ballpark.
https://www.honestjohn.co.uk/realmpg.../c30-2010/16-d
I've got a V60 1.6D, and get around 54mpg. Heavier car, so in theory should be less, but a lot depends on how it is driven as well. Not sure if it's the same engine either.