We present results from molecular dynamics simulations of a confined equimolar binary liquid crystal mixture of generalised Gay-Berne particles with axial ratios of 3.0:1 and 3.5:1. Particle-substrate interactions are based on a nonseparable anisotropic 9-3 form augmented by an azimuth-dependent prefactor. At high and moderate temperatures, the longer particles are preferentially adsorbed at the substrates. Unlike the equivalent 3-d bulk system, however, a significant degree of temperature-driven demixing is observed on cooling, the longer particles migrating into the centre of the cell at low temperatures. This leads to a marked decrease in the surface-region order parameter.