In this paper we present a computer simulation study of thin film behaviour performed using the Lebwohl-Lasher liquid crystal model. In this, we study systems with a slab geometry ofLparallel planes. We have measured the shifts in the bulk transition temperature that result from variation both ofLand of an aligning surface field ϵs. From these, we have used a Kelvin-like equation to calculate the nematic-isotropic surface tension. We have also investigated the nature of the shifted bulk transition in these thin film systems. From detailed comparison of energy distribution functions, we have found that the ϵs= 0 system has a critical thickness, 8 <Lc< 16, below which there is no bulk transition. We note that mean field theories fail to predict this critical point and suggest that its cause is related to the pretransitional divergence in orientational correlations seen at the bulk 3d transition.