We present the results of density functional and quasi-particle calculations on the (110) surface of InP. The density functional calculations are performed in a plane wave basis using ab initio pseudopotentials and constitute the starting point for a simple scheme of quasi-particle correction based upon the GW approach. In contrast to other GW schemes, our quasi-particle corrections are obtained with a minimum of further computation beyond that required for the density functional work. Nevertheless, our quasi-particle results show good agreement with a previous GW calculation and with experiment. Furthermore, our work clearly shows that there is a non-rigid correction to density functional theory which could not be accounted for within any simple scissors operator approach.