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. We focus here on two analogies with the perovskites that have been less thoroughly explored than in the corresponding oxides: changing either the composition, to form a solid solution, or the pressure is likely to distort the structure and hence change the ferroic properties. We propose to study five members of this family ¿ three end members and two solid solutions ¿ by powder diffraction under increased pressure. We will quantify the resistance to distortion of both the individual coordination octahedra and the framework as a whole, in particular monitoring the quenching of Jahn-Teller distortion in d9 Cu2+ ions and its effects on the polarisation and hence ferroic order.