Abstract Nonlinear processes involved in the manufacture of nominally sinusoidal surface relief diffraction gratings can introduce distortions into the profile of these surfaces. Such distortions may dramatically affect both the specular reflectivity and diffracted efficiencies from such a surface, particularly if it is metallic. To illustrate this a comprehensive numerical modelling study of the optical coupling to surface plasmon polaritons on silver gratings has been undertaken. The grating surface profile is represented in terms of a truncated Fourier series, the effect of varying the amplitude, and then the phase, of each Fourier component in turn, is explored. This illustrates the sensitivity of individual features to specific harmonic components of the surface, for surface plasmon resonances recorded in both the zeroth and higher diffracted orders.