Hexagonal YMnO3 is a well-studied, but poorly understood, multiferroic material, which adopts the 'geometric' mechanism of ferroelectric distortion, rather than the more normal mechanisms mediated via the second-order Jahn-Teller effect (the 'd0' or 'lone-pair' mechanisms). Despite several recent high temperature powder diffraction studies, by both X-ray and neutron methods, the nature of the evolution of the crystal structure through the paraelectric-ferroelectric (PE-FE) transition remains ambiguous. Some reports, but not all, suggest an intermediate phase (IP) between the PE and FE phases, with IP probably also being a polar phase. Our aim is to pinpoint the exact structural evolution of this material throughout the range 300 - 1450 K., by exploiting the unique advantages of PND in determining subtle oxygen atoms displacements, together with the high flux and resolution of GEM.