Does anyone know of any common faults with the temp gauge on these?
Other than the sensor, basically it is showing temps of 20 degrees c at the moment which is obviously wrong!
I fitted a new sensor to it and it still seems to be wrong, don't know if it makes any difference but the sensor was in direct sunlight

 

Very common problem on vaux's, sometimes, not always disconnecting the battery can rectify the problem, as you have renewed the sensor which is normally the issue, mine reads up to 75C at the moment on the astra.
tabetha

 

Yeh as above, there was a bullitin from vauxhall out on this for the astra H, when the dash read '--' blank, disconnecting the battery for 5mins and reconnect and test would be fine, this may work in this case, worth a try.