We wish to study the gillespite-structured phase Ba0.5Sr0.5CuSi4O10 at high temperatures to determine the structural basis of the invar-like (zero thermal expansion) of the a-b plane. Our earlier work has shown that the structural response at low temperature is dominated by low frequency lattice modes with Eg symmetry, and a rigid unit mode model has been developed to characterise gillespite structured phases. One aspect of this model has been the prediction that Ba0.5Sr0.5CuSi4O10 would exhibit invar-like behaviour, possibly extending to high temperature (an observation made on BaFeSi4O10 many years ago, but unexplained at the time). We propose to characterise Ba0.5Sr0.5CuSi4O10 in the temperature range 298 K - 1273 K to study the extent of the invar-like behaviour, to characterise deviations from our rigid-mode model, and to examine any tendency towards our proposed aristotype phase.