A one dimensional model of the sheath between the plasma and the wall in a negative ion source has been expanded to include the effects of a variable wall surface work function and fast positive ion species. The plasma consists of positive ions, electrons and negative ions. The model also includes the emission of negative ions from the wall into the sheath and thus represents the conditions in a caesiated ion source with surface production of negative ions. At high current densities of the emitted negative ions, the sheath is unable to support the transport of all the negative ions to the plasma and a virtual cathode is formed. This model takes this into account and allows the calculation of the transported negative ions across the sheath with the virtual cathode. It has been suggested that the work function of caesium can be higher than 1.5eV in certain plasma conditions relevant to H- production. As the negative ion yield has a dependence on the work function there will be an effect on the negative ion current density from the surface and hence the virtual cathode behaviour. Including this effect has shown that in changing the work function from 1.5eV to 2.2eV reduces the maximum transported H- current density by a third.