The anion-excess fluorite (Sr,Y)Cl_2.03 has been studied by coherent diffuse neutron scattering techniques. At ambient temperature the disorder is found to be static within instrumental resolution. The parameters of defect cluster models have been fitted directly to the data. Excess chlorine ions are found to aggregate into cuboctahedral clusters whose ionic coordinates agree with those calculated from a simple hard sphere model. At elevated temperatures the scattering exhibits quasi-elastic energy broadening indicating the dynamic nature of the disorder. Data obtained at 1050 K show that the cuboctahedral clusters have broken up, the scattering being reminiscent of that obtained from pure fluorites in the fast-ion phase. It is possible to account for the high temperature scattering in terms of 'snapshot' models of the diffusing anions and their associated relaxation fields.