A combustion staging system for fuel injectors of a multi-stage combustor of a gas turbine engine, the system including: a splitting unit which receives a metered fuel flow and controllably splits the received fuel flow into pilot and mains flows for injecting respectively at pilot and mains fuel stages of the injectors to perform staging control of the combustor; and pilot (31a, b) and mains fuel manifolds respectively distributing fuel from the splitting unit to the pilot and mains stages; wherein the splitting unit is operable to select the pilot manifold (31a, b) and deselect the mains manifold for pilot-only operation in which there is a pilot flow but no mains flow from the splitting unit, and is operable to select both the pilot (31a, b) and mains manifolds for pilot and mains operation in which there are pilot and mains flows from the splitting unit; wherein the system further includes a cooling flow recirculation line having a delivery section arranged to provide a cooling flow of fuel to the mains manifold when it is deselected during pilot-only operation so that the deselected mains manifold remains primed with relatively cool fuel, and a return section arranged to collect the cooling flow from the mains manifold; wherein the cooling flow enters the delivery section from a high pressure fuel zone of the engine at a cooling flow orifice and exits the return section to a low pressure fuel zone of the engine at a pressure raising orifice; wherein the system further includes a fuel recirculating control valve on the delivery section adjacent the cooling flow orifice, the fuel recirculating control valve having an open position so that the cooling flow enters the delivery section at the cooling flow orifice during pilot-only operation, and a shut off position which prevents the cooling flow entering the delivery section through the cooling flow orifice during pilot and mains operation; characterised in that the system further includes a supplementary valve which is controllable to bleed cooling flow from the recirculation line or feed additional cooling flow into the recirculation line; and the pressure of the cooling flow in the mains manifold is determined by the respective flow numbers of the cooling flow orifice and the pressure raising orifice, and a control setting of the supplementary valve. a splitting unit which receives a metered fuel flow and controllably splits the received fuel flow into pilot and mains flows for injecting respectively at pilot and mains fuel stages of the injectors to perform staging control of the combustor; and pilot (31a, b) and mains fuel manifolds respectively distributing fuel from the splitting unit to the pilot and mains stages; wherein the splitting unit is operable to select the pilot manifold (31a, b) and deselect the mains manifold for pilot-only operation in which there is a pilot flow but no mains flow from the splitting unit, and is operable to select both the pilot (31a, b) and mains manifolds for pilot and mains operation in which there are pilot and mains flows from the splitting unit; wherein the system further includes a cooling flow recirculation line having a delivery section arranged to provide a cooling flow of fuel to the mains manifold when it is deselected during pilot-only operation so that the deselected mains manifold remains primed with relatively cool fuel, and a return section arranged to collect the cooling flow from the mains manifold; wherein the cooling flow enters the delivery section from a high pressure fuel zone of the engine at a cooling flow orifice and exits the return section to a low pressure fuel zone of the engine at a pressure raising orifice; wherein the system further includes a fuel recirculating control valve on the delivery section adjacent the cooling flow orifice, the fuel recirculating control valve having an open position so that the cooling flow enters the delivery section at the cooling flow orifice during pilot-only operation, and a shut off position which prevents the cooling flow entering the delivery section through the cooling flow orifice during pilot and mains operation; characterised in that the system further includes a supplementary valve which is controllable to bleed cooling flow from the recirculation line or feed additional cooling flow into the recirculation line; and the pressure of the cooling flow in the mains manifold is determined by the respective flow numbers of the cooling flow orifice and the pressure raising orifice, and a control setting of the supplementary valve.