I have a 2002 TD6 and with a light right foot have had it displaying 32 mpg (on one occasion) at worst have had it as low as 19 mpg (on autobahn cruising at 100mph) averages daily commute 26 - 28 mpg.
How long have you had it? How have you worked out your figures?
Have you reset your computer display on each trip? in order to double check your own figures
If so your mileage is very poor! As you have only had it for two weeks I'd be taking it back to the garage I bought it from? and get them to check it out!