  Using Monte-Carlo simulation and mean field calculations, we study the
liquid-vapour phase diagram of a square well binary fluid mixture as a function
of a parameter $\delta$ measuring the relative strength of interactions between
particles of dissimilar and similar species. The results reveal a rich variety
of liquid-vapour coexistence behaviour as $\delta$ is tuned. Specifically, we
uncover critical end point behaviour, a triple point involving a vapour and two
liquids of different density, and tricritical behaviour. For a certain range of
$\delta$, the mean field calculations also predict a `hidden' (metastable)
liquid-vapour binodal.

Statistical Mechanics