Numerical simulations of a heavy-duty diesel engine fuelled with n-heptane have been performed with the conditional moment closure (CMC) combustion model and an embedded two-equation soot model. The influence of exhaust gas recirculation on the interaction between post- and main- injection has been investigated. Four different levels of EGR corresponding to intake ambient oxygen volume fractions of 12.6, 15, 18 and 21% have been considered for a constant intake pressure and temperature and unchanged injection configuration. Simulation results have been compared to the experimental data by means of pressure and apparent heat-release rate (AHRR) traces and in-cylinder high-speed imaging of natural soot luminosity and planar laser-induced incandescence (PLII). The simulation was found to reproduce the effect of EGR on AHRR evolutions very well, for both single- and post-injection cases. Further a direct comparison of the computed and measured temporal evolution of soot luminosity is presented for the 18% initial oxygen volume fraction case. The spatial extents of the predicted spray plumes and soot clouds are in qualitative agreement with experimental data. Subsequently, the model has been used to investigate the effect of the post-injection on in-cylinder soot evolution, especially with respect to the role of increased mixing and temperature; a detailed analysis highlighting the relative importance of soot formation and oxidation (both O2 and OH contributions) is presented at two representative EGR levels.