Perovskite-structured MgSiO3 is the dominant mineral in the Earth's lower mantle. The transition between its perovskite (PV) and post-perovskite (PPV) polymorphs controls the structure and dynamics of the thermal boundary layer of the Earth at its core-mantle boundary (CMB). Many properties of MgSiO3 cannot readily be measured at the CMB pressure (~130 GPa) and we have a NERC grant to constrain them by combining measurements on low-P isostructural analogues with ab initio computer simulations of both the analogue and natural systems. We have been awarded time on PEARL to study the crystal structure of PPV-NaCoF3 (one of the best analogues) at high P to ground-truth our simulations. A similar study of PV-NaCoF3 is essential as the contrast in elasticity and deformation mechanisms between the PV- and PPV- phases may define both the structure and seismic properties near to the CMB.