We are able to grow large single crystals of Na0.8CoO2 and to control the sodium superstructure via the cooling rate. Quenching rapidly to low temperature gives a striped phase, whereas slow-cooling results in a square superstructure. Our unpolarised inelastic neutron scattering measurements using IN20 at the ILL show that these superstructures lead to different in-plane exchange interactions. We now propose to use the flexibility of time-of-flight spectroscopy to extend these measurements to higher energies at lower wave-vector transfers on MERLIN at ISIS, in order to determine the dispersion across the Brillouin zone. We have identified a region of reciprocal space with weak phonon scattering that will allow us to determine the in-plane exchange constants. Measurements using the rotation method will be performed for each superstructure above and below the Neel temperature.