Methylammonium lead iodide has recently been recognised as an excellent photovoltaic material, with energy conversion efficient of over 20% with the prospect of gaining higher efficiencies. The crystal structure at ambient shows two phase transitions involving ordering of the orientation of the polar methylammonium molecular cation and tilts of the PbI6 octahedra. Recently there has been a report of a high-pressure phase transition to a completely new phase. We plan to study this phase transition, exploiting the fact that neutron diffraction can give detail on this system (such as concerning the orientation of the methylammonium molecular cation) that cannot be obtained by x-ray diffraction. Understanding the structural flexibility of this material under changing conditions (temperature and/or pressure) is important to support future developments.