Simulated data sets to investigate the effect on the position and contrast ratio of a point source at 20 mas. Each contains an upper and a lower panel displaying how the fitted separation (upper) and contrast ratio (lower) depends upon different disc parameters. Top left panel: varying point source contrast ratio (model – blue circles, best fit – green triangles). Top right panel: varying disc contrast, f_disc ∕f_total. Bottom left panel: varying disc major-axis position angle. Bottom right panel: disc semi-major axis. In the upper panels, the dashed and solid black lines represent λ ∕B and λ ∕2B resolution limits of the uv-plane, respectively. λ∕2B (~26 mas) represents the minimum separation for which we search for a companion. Within the lower panels, the solid black line represents the true contrast ratio of the point source in the absence of a disc. We never find a point source at this contrast ratio as a result of the minimum separation limits we place on the fitting routine. At smaller separations the algorithm struggles to find a minimum in the χ². We find that the separation is over estimated in cases where the position angle of the point source and of the disc semi-major axis are approximately orthogonal and where the disc radius is such to produce an enhancement to the closure phase signal at short baselines.