Colloidal suspensions are a class of fluid in which microscopic particles are dispersed in a liquid solvent. Examples arise in a host of natural and synthetic soft matter systems including micelles, protein solutions, oil-water emulsions, liquid crystals and blood. Technologically, colloidal suspensions feature in applications as diverse as paints, drug delivery systems and lubricants. They are also prevalent in consumer items such as detergents and cosmetics as well as many foodstuffs like mayonnaise. However, in all such systems one issue is key, namely the stability of the suspension, or more generally its phase behaviour.This work will employ computer simulation to study the properties of dispersions of colloidal particles to which much smaller nanoparticles have been added. Such additives have been shown to dramatically change the physical properties of a dispersion, specifically the tendancy of the colloidal particles to phase separate or to aggregate into a gel. This happens because the additive modifies the effective interaction betwen colloid particles. However, a complete understanding of how this happens and exactly how it depends on the colloid-additive and additive-additive interactions is presently lacking for anything other than hard sphere systems. We shall address this issue in the present work using computer simulations of simplified models namely highly size-asymmetric binary fluid mixtures interacting via dispersion and coulomb forces. Until recently, such studies were technically too difficult to perform, but a new simulation method developed by the proposer now offers direct access to problems of real practical interest. We plan to systematically elucidate the effects of various types and concentrations of additive on phase behaviour and to discover the conditions under which exotic self-assembled inhomogeneous phases occurs, such as modulated structures or cluster phases. Such structured phases potentially offer great utility as templates for nanolithography, nanoelectronics, photonic crystals and protein crystallization.