A combustion staging system for fuel injectors 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 injectors; a splitting unit which controllably splits a metered fuel flow and sends respective portions of the metered flow to the pilot and mains fuel manifolds to perform staging control of the combustor, the splitting unit being able to deselect the mains manifold so that there is no flow into the combustor from the mains manifold; 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: at least a portion of the return section of the cooling flow recirculation line which is angled so that the cooling flow of fuel runs downhill in that portion of the return section; a first fuel pressure sensor which detects the pressure of the cooling flow on the return section, the first pressure sensor detecting the pressure of the cooling flow at a location downstream of the downhill angled portion; and a control arrangement which is adapted to close off the recirculation line when the first pressure sensor indicates a failure of the cooling flow. pilot and mains fuel manifolds respectively distributing fuel to pilot and mains stages of the injectors; a splitting unit which controllably splits a metered fuel flow and sends respective portions of the metered flow to the pilot and mains fuel manifolds to perform staging control of the combustor, the splitting unit being able to deselect the mains manifold so that there is no flow into the combustor from the mains manifold; 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: at least a portion of the return section of the cooling flow recirculation line which is angled so that the cooling flow of fuel runs downhill in that portion of the return section; a first fuel pressure sensor which detects the pressure of the cooling flow on the return section, the first pressure sensor detecting the pressure of the cooling flow at a location downstream of the downhill angled portion; and a control arrangement which is adapted to close off the recirculation line when the first pressure sensor indicates a failure of the cooling flow. at least a portion of the return section of the cooling flow recirculation line which is angled so that the cooling flow of fuel runs downhill in that portion of the return section; a first fuel pressure sensor which detects the pressure of the cooling flow on the return section, the first pressure sensor detecting the pressure of the cooling flow at a location downstream of the downhill angled portion; and a control arrangement which is adapted to close off the recirculation line when the first pressure sensor indicates a failure of the cooling flow.