sounds a tad low tbh but basics first.
95 or 98/99 RON fuel?
general usage over what distance/speed etc (like Steve has asked)
when was the car last serviced?
all tyre pressures ok?
air con on all the time? + do you have any extras that may impact mpg? (e.g. stereo systems)
from their its health checks on the car DV/PCV etc , maybe including VCDS logging to check parameters for things like boost etc are ok
Is the car totallly standard ? has it always had poor mpg?
a quick check is to find a flat road and check the instantaneous consumption figure in 6th @70mph