The metal formate perovskites have structural chemistry as diverse as their inorganic analogues; yet substantial areas of their phase diagrams, including phases with potentially useful functionality, remain unexplored. Using I19 data, we recently observed a low-temperature incommensurately modulated phase in methylammonium manganese formate and solved its structure. This will be the first modulated structure reported in this family and offers a potential novel route to achieving electrical order. Here we seek to measure how the modulation varies with temperature, including any lock-in to a commensurate structure; and to understand the differences between this material and the cobalt analogue, which has quite different low-temperature phase behaviour. Combining single-crystal X-ray and high resolution powder neutron data is a very powerful way to shed light on subtle structural changes.