A combustion staging system for fuel burners of a multi-stage combustor of a gas turbine engine, the system having: pilot and mains fuel manifolds respectively distributing fuel to pilot and mains stages of the burners; a plurality of check valves through which the mains manifold feeds fuel to the respective burners, the check valves being arranged to open when the fuel pressure within the mains manifold exceeds a predetermined fuel pressure relative to gas pressure in the combustor; a splitting unit which receives a metered fuel flow and controllably splits the received fuel flow into pilot and mains flows which are sent respectively to the pilot and mains fuel manifolds to perform staging control of the combustor, the splitting unit being operable to select the pilot manifold and to 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 burners, and being 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 burners; and a cooling flow recirculation line having a delivery section arranged to provide a cooling flow of fuel to the mains manifold when that manifold is deselected 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; characterised in that the system further has: a gas pressure sensor configured to measure gas pressure in the combustor; a speed sensor which measures a rotation speed of a rotor of the engine; and a control unit which, when the mains manifold is deselected for pilot-only operation, (i) compares the measured rotation speed with a predicted rotation speed for the metered fuel flow, and compares the measured gas pressure with a predicted gas pressure for the metered fuel flow, and (ii) is adapted to close off the recirculation line when the results of these comparisons are indicative of leakage of fuel from the cooling fuel flow into the combustor through one or more of the check valves. pilot and mains fuel manifolds respectively distributing fuel to pilot and mains stages of the burners; a plurality of check valves through which the mains manifold feeds fuel to the respective burners, the check valves being arranged to open when the fuel pressure within the mains manifold exceeds a predetermined fuel pressure relative to gas pressure in the combustor; a splitting unit which receives a metered fuel flow and controllably splits the received fuel flow into pilot and mains flows which are sent respectively to the pilot and mains fuel manifolds to perform staging control of the combustor, the splitting unit being operable to select the pilot manifold and to 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 burners, and being 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 burners; and a cooling flow recirculation line having a delivery section arranged to provide a cooling flow of fuel to the mains manifold when that manifold is deselected 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; characterised in that the system further has: a gas pressure sensor configured to measure gas pressure in the combustor; a speed sensor which measures a rotation speed of a rotor of the engine; and a control unit which, when the mains manifold is deselected for pilot-only operation, (i) compares the measured rotation speed with a predicted rotation speed for the metered fuel flow, and compares the measured gas pressure with a predicted gas pressure for the metered fuel flow, and (ii) is adapted to close off the recirculation line when the results of these comparisons are indicative of leakage of fuel from the cooling fuel flow into the combustor through one or more of the check valves. a gas pressure sensor configured to measure gas pressure in the combustor; a speed sensor which measures a rotation speed of a rotor of the engine; and a control unit which, when the mains manifold is deselected for pilot-only operation, (i) compares the measured rotation speed with a predicted rotation speed for the metered fuel flow, and compares the measured gas pressure with a predicted gas pressure for the metered fuel flow, and (ii) is adapted to close off the recirculation line when the results of these comparisons are indicative of leakage of fuel from the cooling fuel flow into the combustor through one or more of the check valves.