A liquid level detector for use in a pump control system comprises a chamber 20 for liquid, a liquid inlet 6 to the chamber 20, a liquid outlet 7 from the chamber and connectable to a pump, and a capacitive sensor comprising mutually spaced capacitive sensor elements (21, 22; 22, 23) forming a capacitance which is sensitive to permittivity within a region of the chamber proximate the capacitive sensor. The chamber is defined at least partially by a barrier member 25 and the capacitive sensor elements are provided on the barrier member outside the chamber.