We have recently fully determined the root-5 and root-2 vacancy superstructures of the insulating and superconducting phases of Cs0.8Fe1.6Se2 using neutron diffraction and density-functional theory (DFT). On the basis of our DFT calculations we predict a ferrimagnetic structure for the root-5 phase and a block antiferromagnet for the root-2 phase at low temperature. We now plan to search for the predicted magnetic ordering using polarised neutrons on IN20. The DFT calculations also reveal strong spin-phonon coupling, and we plan to separate these excitations using polarized inelastic neutron scattering on IN20. For the root-5 phase we shall search for predicted changes to both the phonon and magnon dispersions when transforming from the antiferromagnetic to ferrimagnetic phases. In the case of the root-2 phase we shall search for magnetic excitations emerging out of the superlattice reflections, and at the zone boundaries where a magnetic resonance has previously been observed in the superconducting phase.