The iron-based superconductors AxFe2-ySe2, where A = K, Rb and Cs, are remarkable for the coexistence of large-moment magnetism with superconductivity. We have measured the superstructure of superconducting Cs0.8Fe1.6Se2 and its associated magnetic ordering using unpolarised neutrons on SXD at ISIS. We find evidence for canting of the moments away from the c-direction for the expected block antiferromagnetic structure. Furthermore, on the basis of our density-functional calculations, we predict a small ferromagnetic component at low temperature. We now propose to determine the magnetic structures using neutron diffraction with uniaxial polarisation analysis on either IN20 or D3. By applying a vertical field in the c-direction we shall obtain a single magnetic domain. We shall have great sensitivity to the development of a small ferromagnetic component through the interference term between structure and magnetism in the non-spin-flip channel, and to the canting of the moments away from the c-direction in the spin-flip channel. [This is a much more focussed resubmission to study the most important aspect of 4-01-1356: the magnetic structure determination.]