A liquid suction pump, the pump comprising: a drive pipe to receive a liquid drive flow for the pump; a liquid conduit having first and second liquid delivery arms (13, 14) to provide pumped liquid, and a connecting valve arrangement between the arms (13, 14); first and second pump inlets (16, 17) to said first and second arms (13, 14), said first and second pump inlets (16, 17) having respective first and second one-way inlet valves; said valve arrangement having a valve inlet coupled to said drive pipe and valve outlets coupled to said first and second arms, to alternately close off a liquid connection between said valve inlet and respective ones of said first and second arms; and a compliant element coupled to said drive pipe; wherein the suction pump is configured such that, in operation, said drive flow oscillates in pressure/flow rate due to alternate switching of said valve arrangement; and wherein a compliance of said compliant element is such that a geometry of said suction pump in combination with said compliance defines a resonant condition for said pump and said oscillation is at a resonant frequency of the pump, characterised in that: said oscillation causes said switching, and the amplitude of a pressure variation of said resonant oscillation, at said valve arrangement, is sufficient to switch said valve arrangement between alternate positions when said pressure is at a minimum, and an amplitude of the pressure variation in or at the compliant element is equal to or greater than a differential in pressure across the valve arrangement between the valve inlet and a closed-off valve outlet. a drive pipe to receive a liquid drive flow for the pump; a liquid conduit having first and second liquid delivery arms (13, 14) to provide pumped liquid, and a connecting valve arrangement between the arms (13, 14); first and second pump inlets (16, 17) to said first and second arms (13, 14), said first and second pump inlets (16, 17) having respective first and second one-way inlet valves; said valve arrangement having a valve inlet coupled to said drive pipe and valve outlets coupled to said first and second arms, to alternately close off a liquid connection between said valve inlet and respective ones of said first and second arms; and a compliant element coupled to said drive pipe; wherein the suction pump is configured such that, in operation, said drive flow oscillates in pressure/flow rate due to alternate switching of said valve arrangement; and wherein a compliance of said compliant element is such that a geometry of said suction pump in combination with said compliance defines a resonant condition for said pump and said oscillation is at a resonant frequency of the pump, characterised in that: said oscillation causes said switching, and the amplitude of a pressure variation of said resonant oscillation, at said valve arrangement, is sufficient to switch said valve arrangement between alternate positions when said pressure is at a minimum, and an amplitude of the pressure variation in or at the compliant element is equal to or greater than a differential in pressure across the valve arrangement between the valve inlet and a closed-off valve outlet.