A fuel supply system for fuel injectors of a multi-stage combustor of a gas turbine engine, the system having: a metering and splitting arrangement which receives a pressurised fuel flow and controllably meters and splits the received flow into metered pilot and mains flows for injecting respectively at pilot and mains fuel discharge orifices of the injectors to perform staging control of the combustor; pilot (31a, 31b) and mains fuel distribution pipeworks respectively distributing fuel from the metering and splitting arrangement to the pilot and mains discharge orifices; and a controller (43, 45) which is configured to command the metering and splitting arrangement to provide the metered flows, to select the pilot distribution pipework (31a, 31b) and deselect the mains distribution pipework for pilot-only operation in which there is a pilot flow to the combustor but no mains flow to the combustor from the injectors, and to select both the pilot and mains distribution pipeworks for pilot and mains operation in which there are pilot and mains flows to the combustor from the injectors; wherein: the system further has a pressure sensor configured to detect pressure and provide the detected pressure to the controller (43, 45); the system further has priming means for providing a priming flow to the mains distribution pipework; and characterized in that the controller (43, 45) is further configured to command the priming means to provide the priming flow to the mains distribution pipework when pilot and mains operation is initiated in order to refill, preliminary to providing the mains flow, a volume of the mains distribution pipework adjacent the mains discharge orifices from which fuel drains when the mains distribution pipework is deselected for pilot-only operation, and to command the priming means to stop the priming flow when the detected pressure is indicative of a pressure rise caused by combustion of the priming flow. a metering and splitting arrangement which receives a pressurised fuel flow and controllably meters and splits the received flow into metered pilot and mains flows for injecting respectively at pilot and mains fuel discharge orifices of the injectors to perform staging control of the combustor; pilot (31a, 31b) and mains fuel distribution pipeworks respectively distributing fuel from the metering and splitting arrangement to the pilot and mains discharge orifices; and a controller (43, 45) which is configured to command the metering and splitting arrangement to provide the metered flows, to select the pilot distribution pipework (31a, 31b) and deselect the mains distribution pipework for pilot-only operation in which there is a pilot flow to the combustor but no mains flow to the combustor from the injectors, and to select both the pilot and mains distribution pipeworks for pilot and mains operation in which there are pilot and mains flows to the combustor from the injectors; wherein: the system further has a pressure sensor configured to detect pressure and provide the detected pressure to the controller (43, 45); the system further has priming means for providing a priming flow to the mains distribution pipework; and characterized in that the controller (43, 45) is further configured to command the priming means to provide the priming flow to the mains distribution pipework when pilot and mains operation is initiated in order to refill, preliminary to providing the mains flow, a volume of the mains distribution pipework adjacent the mains discharge orifices from which fuel drains when the mains distribution pipework is deselected for pilot-only operation, and to command the priming means to stop the priming flow when the detected pressure is indicative of a pressure rise caused by combustion of the priming flow. the system further has a pressure sensor configured to detect pressure and provide the detected pressure to the controller (43, 45); the system further has priming means for providing a priming flow to the mains distribution pipework; and characterized in that the controller (43, 45) is further configured to command the priming means to provide the priming flow to the mains distribution pipework when pilot and mains operation is initiated in order to refill, preliminary to providing the mains flow, a volume of the mains distribution pipework adjacent the mains discharge orifices from which fuel drains when the mains distribution pipework is deselected for pilot-only operation, and to command the priming means to stop the priming flow when the detected pressure is indicative of a pressure rise caused by combustion of the priming flow.