Negative linear compressibility (NLC) is the phenomenon whereby at least one of the linear dimensions of a material actually expand under hydrostatic pressure. It is only recently that the effect has been observed to any serious degree, and very recently we have discovered KMn[Ag(CN)2]3 shows NLC to an order of magnitude greater than any other material. Our data on which this discovery was based were not of sufficient quality to enable unambiguous structure refinement as a function of pressure, and so we cannot yet be definitive in our understanding of the mechanism of NLC in this material. Here we are proposing to perform a definitive combined variable-pressure / variable-temperature neutron diffraction experiment to characterise the structural changes in KMn[Ag(CN)2]3 under pressure, and to determine the interplay between its NLC and negative thermal expansion (NTE) properties.