Neutron reflectivity offers the opportunity for non-destructive characterisation of buried polymer interfaces. Specular neutron reflectivity is commonly used to characterise the structure of conjugated polymer interfaces found in organic electronic devices. However, detecting specularly reflected neutrons only allows the measurement of the root-mean-square (RMS) roughness of the interface averaged over a macroscopic lateral distance. There are two contributions to the RMS roughness of a polymer/polymer interface; i) the so-called 'intrinsic interfacial width' due to mixing of the polymers at a molecular level and ii) lateral roughness due to deviations of the interface position from the plane of the substrate. In this work a numerical model is developed to analyse experimental off specular reflectivity. The model is based on the distorted wave Born approximation (DWBA). Specular and off specular neutron reflectivity data is collected from a model conjugated polymer/amorphous polymer interface (poly(9,9-dioctylfluorene) (F8) on deuterated PMMA) and a conjugated poly- mer/fullerene interface (poly(3-hexylthiophene) (P3HT) polymer on [6,6]-phenyl C61-butyric acid methylester (PCBM)). This allows probing of the structure in the plane of the interfaces to distinguish the intrinsic interfacial width from the lateral roughness. The structure of the F8/dPMMA interface is studied by systematically varying the film thickness, which strongly impacts on the amplitude of the lateral interface roughness, and allows more complete analysis of the relative contributions of intrinsic mixing and lateral roughness. For comparison off specular measurements on amorphous/amorphous polymer (PMMA/polystyrene) interfaces are performed, which have been studied previously using specular neutron reflectivity and self consistent held theory. Fitting specular reflectivity using standard techniques and off specular reflectivity data using the model developed allow intrinsic and lateral roughness contributions for the F8/dPMMA system to be separated by direct measurement. The P3HT/PCBM interface exhibits no lateral roughness as the materials are found to be miscible.