Several distinct families of layered perovskites have been shown experimentally to display ferroelectricity. The most recent of these is the Dion-Jacobson (DJ) family, typified by compositions such as CsBiNb2O7, which has now been shown to be ferroelectric with a TC of ~ 1033 C. Simultaneously, theoretical studies have suggested that ferroelectricity in the DJ family arises from a 'trilinear coupling', whereby two non-polar lattice modes interact in such a way to cooperate with a third, polar mode. Both these studies were prompted by our own earlier work, which predicted ferroelectricity in CsBiNb2O7 based on a crystallographic study at ISIS. Unfortunately, our earlier VT study only extended to 900 C; with the benefit of hindsight(!) it is clear that a further study is prompted in order to clarify the exact nature of the ferro-paraelectric transition in this and related compounds.