A number of relaxor ferroelectric ceramics possess a near-stable relative permittivity over a wide temperature range, making them promising materials for electronic applications under extreme conditions. The physical basis of temperature stability is not explained by current theory. Electron microscopy suggests that local phase segregation produces polar and weakly polar nano-regions in the parent crystal lattice. These regions restrict nano-polar growth and ordering on cooling, maintaining the flat temperature response. We probe this local phase segregation in a perovskite solid solution series, (1-x)(Ba0.8Ca0.2)TiO3-xBi(Mg0.5Ti0.5)O3, across the temperature range of stable permittivity (150-500°C). Neutron PDF will probe the T-dependent changes in the local atomic structure. The aim is to obtain a nanostructure model for theoretical modelling and prediction of permittivity.