We propose a single crystal neutron diffraction study of the nuclear and magnetic structure of the quadruple perovskite (LaMn3)Mn4O12. Our motivation is a very recent observation of ferroelectricity with a polarisation P ~ 1 uC/cm2 induced by a magnetic order, to the best of our knowledge the largest value hitherto reported for an improper ferroelectric. This prompts us to investigate (i) the magnetic structure and (ii) the expected breaking of the centre of symmetry at the magnetic ordering transition. This shall enable us to elucidate the role of the exchange striction driving the ferroelectricity and stimulate the search for novel multiferroics with optimised properties.