faulty thermostat??

[ben10]

Hi fellow passatians, hopefully an easy one; the needle on the temp gauge in my B5 tdi 110 afn only goes up to just under quarter way, even when sat in traffic or doing 80 (ish) on the motorway, and the heaters only blow
warmish, so is this the sign of a dead thermostat or is it the temp sensor?

A recent fault code read didn't pick up any codes for temp sensor (not vag-com)

I'm still getting 55mpg ave. on my 8 mile trip to work so seems to be running ok, if it is the thermostat will it be wise to get the thermostat housing too as i've read about the plastic fingers breaking? or is it a case of just being careful when removing the old thermostat?

thanks

Ben