A mean-field theory is presented which describes the basic observations of recent experiments revealing rich Wetting behaviour of n-alkane/methanol mixtures at the liquid-vapour interface. The theory is based on a microscopic lattice-gas model from which a Cahn-Landau phenomenological approach is derived. Besides the physics associated with the short-range components of the intermolecular interactions, the effects of them long-range tails of the net van der Waals forces between interfaces are also taken into account. The tricritical wetting transition, which is featured in the theory when long-range forces are ignored, is replaced by a critical prewetting transition when these forces are included. The experimental observations agree to a large extent with the predictions of the theory for short-range tricritical wetting.