The dash mpg displays aren't renowned for their accuracy so unless you've had a chance to compare the gauge with the range over a few tankfuls range between fills is a better indication.
Suggest you follow Rob's guidelines on the range and if you haven't used an OEM (Bosch) sensor. You might keep an eye on temp gauge to see how long it takes to warm up. If the thermostat is stuck open the engine will take a long time to warm up and until it does mpg will take a battering.
You don't say what sort of driving your doing but although 30 is low if your doing short local journeys it's not going to get stellar mpg in the winter months.
I've a 1.9 Mk5 Tdi 105PS (had it 6 years) and know my display mpg is pretty accurate.On short local trips round town it takes a while to get to 30 (whilst the engine warms) and at the weekend on a 35mile round trip (couple of hour break) of mixed town/urban A+B roads got 42mpg
Summer on long A+motorway high 50-mid 60