A thermally sensitive control (2) is provided in or for use in a liquid heating vessel having a sheathed electric heater (92) mounted to a diffuser plate (80) provided on its base (8). The control comprises first (10) and second (12) actuators arranged to make good thermal contact with the vessel base. Biasing means (20) are arranged to exert a variable biasing force on the second actuator (12) so as to vary its operating temperature in use. The actuator (12) engages a region (84) of the diffuser plate (80) which is surrounded by (3) spaced slots (86), (88), and (90) which act to isolate the region (84) from the heater (92) so that its temperature reflects more accurately the temperature of the base of the vessel and that of the liquid being heated inside the vessel.