Incommensurate magnetic phases generated by frustrated magnetic exchange interactions can stabilise novel induced magnetoelectrics. We have recently found that the two-dimensional oxide beta-NaMnO2 exhibits this kind of magnetoelectricity. Bulk magnetodielectric (MD) measurements and preliminary constant wavelength diffraction data(NPD) find two phase transitions: from a paramagnetic to a high-T commensurate magnetic phase (200 K) the system transforms to an incommensurate state at lower temperatures (100 K). The complexity of the magnetic structure as seen in our NPD profiles, requests the higher resolution of WISH at long d-spacings. We propose to collect Rietveld refineable data around the two magnetic phase transitions. Solving the magnetic structure will enable us to understand the key role of the spin configuration in the mechanism that generates the magnetoelectric coupling.