The pre-transitional Kerr effect in K18 is examined using a novel technique. Mixed polarisation grating coupled guided modes are excited within the liquid crystal layer, these are probed for small changes in position using a differential technique which resolves induced birefringences to an accuracy of 5 × 10−8. The experiment is performed at several different temperatures in order to characterize this pre-transitional behaviour, and from this, one of the Landau expansion coefficients can be found.