This is an experiment to correlate the crystal structures, compositions and physical properties of a series of layered iron selenide hydroxide superconductors synthesised using soft chemistry. Neutrons are required to locate light atoms accurately and refine compositions with a high degree of precision as well as to identify possible magnetic scattering. The results of the experiment will be a clear understanding of the factors that govern superconductivity in iron selenides and the range of available compositions in this class of compound.