The family of guanidinium metal formates has the perovskite structure and, like many perovskites, is of interest for these materials' magnetic and electric properties, in particular the possibility of multiferroicity. The phase diagrams of these compounds, however, remain largely unexplored. We have recently (RB1410586) collected PEARL data that demonstrate two different high-pressure phases in members of this family. However, although we have successfully solved the structure of one of these, the high-pressure phase of copper guanidinium formate has a large unit cell with low (apparently monoclinic) symmetry, causing substantial peak overlap and preventing us from solving the structure. We therefore propose to take advantage of WISH's optimisation for long d spacings to measure low-angle data and hence complete our characterisation of this new phase.