We report experimental observations on the way that flowing polyethylene melts can crystallise within a processing channel geometry. Using a recently developed Multipass Rheometer (MPR), we present rheological, rheo-optic and coupled X-ray data that follow the evolution of crystallisation, as molten polyethylene flows into a slit geometry. Optical observations show that fibrous crystallisation occurs initially at the walls of the slit and not, as expected, in the entrance region to the slit. The coupled X-ray, rheology and rheo-optic data lead us to speculate that a coil-stretch transition of the polymer chains occurs at the wall of the slit and this acts as the primary cause of fibrous X-ray nucleation. At high wall shear rates we identify evidence to suggest that slip occurs between the flowing polymer and the solid wall and this in turn causes the onset of fibrous crystallisation to be surpressed. The experimental observations are generally consistent with certain theoretical predictions made by Brochard and de Gennes.