The hard hexagon adsorption isotherm of Baxter and Joyce was previously used in a statistical model for the underpotential deposition (upd) of Cu on Au(111). This model gave a plausible physical picture of Cu upd on Au(111) and was able to reproduce the gross features of the voltammogram. In the present work the hexagon model is extended to include a mean field treatment of the adsorbate interactions. Using this extended hexagon model, coupled adsorption isotherms for copper and sulfate ions are obtained and solved numerically. These isotherms yield theoretical quartz microbalance curves which compare well with the experimental ones. The model voltammogram reproduces the experimental voltammogram including the finer details of the broad foot region.