We have a long established interest in the materials used for the immobilisation of nuclear waste from the domestic power industry and legacy waste from various defence programmes. We are currently investigating the site substitution preferences for various surrogate waste cations in beta-tricalcium phosphate, using a range of techniques, in order to understand the limits on loading of nuclear waste in this host. In the case of Mg2+ substitution, the evidence from XRD and NMR is that the Mg2+ cations cause a contraction of the unit cell, however the exact nature of the substitution is unknown due to conflicting evidence from various sources. We wish to perform total scattering experiments to conclusively understand the nature of the Mg2+ substitution.