FeSe is a superconductor below 8.5 K. Very recently, we have found superconductivity in a wide compositional range in a series of layered lithium iron selenide hydroxides. Control of thereactions shows a correlation of the occurrence of superconductivity and increasing Tc, up to 39 K, with the minimisation of the concentration of iron vacancies in the iron selenide layer, whichsimultaneously increases the electron count on iron in the selenide layers. This experiment will extract (a) the behaviour of the superfluid stiffness as we vary Tc controllably in this series; (b) the development of the magnetism as composition is varied.