Electric thermometers are called thermocouples, the way they work is that they offer less resistance to electrical current as temperature goes up, i.e. as it gets hotter the voltage on the circuit goes up. Themrocouples are calibrated to give specific voltages at specific temperatures, the temperature readout is simply an indication of the voltage on the circuit.
-40 looks like the lowest possible reading, so if you know it isn't right that indicates there is no voltage on the circuit, so most likely the sensor has failed or the wiring for the sensor has failed. I don't know vauxhalls, but a quick search shows that loads of people replace the sensor (in the front bumper) and they still have problems, so it would be worth your while to check the wiring for the sensor before spending hard-earned wonga on it.