The boiling point will change as the distillate is removed. It is this that I wish to control as it should boil continuously but not be allowed to boil too violently.
In that case, you don't want to control the temperature. The temperature is set by the boiling point of the liquid, and that changes.
You want to control the rate of evaporation. If you measure the flow of vapour away from the liquid, that can be controlled by varying the heat input. You could measure the vapour flow by feeding it though a slight restriction and measuring the pressure drop, or by seeing how much the cooling water in the condenser heats up (if you have a condenser!)
Auto-shut off electric kettles have the temperature sensor set at about 50 deg C, but put in the vapour above the liquid. When the kettle boils, there is suddenly a lot of vapour and that heats the sensor, so the kettle shuts off. They don't try to measure the water temperature, because it wouldn't work. Slight manufacturing or altitude variations would mean that some kettles would never boil and others would never turn off.
You could measure the rate of rise of temperature. That would need a microcontroller. When the rate of rise was over a certain value, the immersion heater should be full on. Otherwise, it should be at a low power. When the liquid is heating up, the rate of change would keep the immersion heater turned on. As it boils, the temperature stops going up and the power is reduced.