The magnetic moments in our model lie within the ac-plane, however components along the b-direction would be permitted by symmetry. The presence of a component along b would result in intensity at the 011 mag. peak position (Q = 1.00 Å -1 ) which is not seen in our data and so any non-collinearity must be small, θ < 8 • . The background of this subtracted I 1.5K -I 30K dataset contains a broad negative feature characteristic of magnetic diffuse scattering, which could be modelled by a broad Lorentzian peak centred at the 101 mag.