I managed to order one from Asko Australia. In case anyone’s after one in the future, the thermistor is part no. 8801474 which costs $29.33 plus $15 (!!) shipping.
I temporarily replaced the thermistor with a fixed resistor and the heater came on, so it’s definitely the thermistor. Hopefully I can find a replacement somewhere!
Just a quick update, the original thermistor (part 8076102) has been replaced by part number 8801474. The spare parts catalogue for the Asko 6000 series on guaranteedparts.com lists the thermistor’s resistance as 6.1 – 3.8K, which is much higher than my 118 ohms. Time to look for a replacement!