We address the detailed atomistic mechanism of ferroelectricity in a new family of oxides typified by the composition CsNbW2O9. Contrary to the mechanism in most ferroelectrics, the characteristic electrical polarisation is not caused spontaneously by polar displacements of ionic species in the crystal, but arises as a secondary consequence of a non-polar lattice instability (so-called 'improper' ferroelectricity). These compounds therefore have something in common with the well-known improper ferroelectric YMnO3, but the similarity is based on symmetry considerations (they are both hexagonal crystals) rather than any direct chemical or structural similarity. We propose to study the detailed nature of this new family of improper ferroelectrics, particularly the transition into the ferroelectric state and the crystallographic features that drive it, for which HRPD is essential.