We have demonstrated, for the first time, that the polarisation of gamma rays in the MeV regime (< 74 MeV) can be measured using a novel detection technique, namely tracking the gamma-ray conversion pairs using a gaseous time projection chamber (TPC). HARPO (the Hermetic Argon Polarimeter) is, to date, the only instrument to have successfully carried out this measurement. Having demonstrated that a TPC can be used to detect and measure the polarisation of MeV gamma rays, we have begun a new, larger study with the goal of flying a TPC on a balloon to validate its preformance in a background-dominated environment. We will describe the mission concept for this gamma-ray polarimeter and also the science that can be addressed both with this demonstrator and with an ultimate, satellite-based instrument.