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 total 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 and deselect the mains manifold for pilot-only operation in which there is a pilot supply to the combustor but no mains supply to the combustor from the injectors, and is operable to select both the pilot and mains manifolds for pilot and mains operation in which there are pilot and mains supplies to the combustor from the injectors; wherein the system further includes a mains connection pipe which delivers fuel from the splitting unit to the mains fuel manifold; 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, the return section including a portion of the mains connection pipe; wherein the cooling flow enters the delivery section from a high pressure fuel zone of the engine and exits the return section to a low pressure fuel zone of the engine; and wherein the system further includes a controller, characterised in that: the return section further includes a branch pipe from the mains connection pipe; and the controller is configured to adjust the splitting unit during pilot-only operation to partially select the mains manifold so that a portion of the total metered flow received by the splitting unit is fed into the cooling flow recirculation line to supplement the cooling flow, thereby increasing the pressure in the mains manifold to meet a target fuel pressure therein. a splitting unit which receives a total 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 and deselect the mains manifold for pilot-only operation in which there is a pilot supply to the combustor but no mains supply to the combustor from the injectors, and is operable to select both the pilot and mains manifolds for pilot and mains operation in which there are pilot and mains supplies to the combustor from the injectors; wherein the system further includes a mains connection pipe which delivers fuel from the splitting unit to the mains fuel manifold; 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, the return section including a portion of the mains connection pipe; wherein the cooling flow enters the delivery section from a high pressure fuel zone of the engine and exits the return section to a low pressure fuel zone of the engine; and wherein the system further includes a controller, characterised in that: the return section further includes a branch pipe from the mains connection pipe; and the controller is configured to adjust the splitting unit during pilot-only operation to partially select the mains manifold so that a portion of the total metered flow received by the splitting unit is fed into the cooling flow recirculation line to supplement the cooling flow, thereby increasing the pressure in the mains manifold to meet a target fuel pressure therein. the return section further includes a branch pipe from the mains connection pipe; and the controller is configured to adjust the splitting unit during pilot-only operation to partially select the mains manifold so that a portion of the total metered flow received by the splitting unit is fed into the cooling flow recirculation line to supplement the cooling flow, thereby increasing the pressure in the mains manifold to meet a target fuel pressure therein.