How do I fix an E20 Error on Hotpoint Dishwasher 7885?

My Hotpoint Dishwasher has developed an intermittent fault coming up with an E20 error code at randon (sometimes within seconds of starting the cycle, sometimes part-way through the wash). From what I can find out, E20 means 'Thermister Under temperature' but the water is getting warm so the heater must be OK. Since the E20 codes comes up sometime during the initial draining at the start of the cycle, I have to assume that the water temperature isn't the fault.

Anyone know to fix this or diagnose further?