Multiferroic materials, where there is a strong coupling between magnetic and electric order parameters, offer a route to novel voltage-controlled spintronics. Y-type hexaferrites are a well-known multiferroic systems, exhibiting multiferroic phases in externally applied magnetic fields, and offer the chance of device applications if a phase exhibiting a room-temperature multiferroicity in low externally applied field could be stabilised. Here we propose to study the magnetic structure of such a candidate system, BaSrMg2Fe12O22, which we recently discovered. In order to map the low-field magnetic phase diagram fully, we propose to perform a neutron powder diffraction experiment on WISH. We require two days of experimental time; our results will represent the first comprehensive study of the low-field magnetic order of this new hexaferrite.