Optical extinction angle spectra of a pyrimidine smectic c material in the surface stabilised geometry have been measured as a function of applied ac electric field and temperature, The results were fitted using a model which assumes uniformly tilted layers either side of a infinitely anchored chevron interface. The model presented uses the free energy expression of Leslie et. al.(1) to calculate the liquid crystal c-director profile through the cell and Jones calculus to determine the associated optical properties. With no applied voltage this gives a structure very similar to a triangular director profile An applied ac voltage couples to the dielectric biaxiality causing director reorientation in the bulk of the cell, At increased voltages an excellent fit is obtained by allowing movement of the director at the surface, This surface movement is described by inclusion of finite surface anchoring in the model, Tile fits give the c-director bend (B-1) and splay (B-2) elastic constants and the effective surface anchoring as a function of temperature.