We have developed an atom-based Monte Carlo simulation model that gives rise to a nanoscale polar domain structure as envisaged to occur in PbZn1/3Nb2/3O3 (PZN) and similar relaxor ferroelectric materials. Individual domains are essentially thin plate-like domains normal to each of the six <110> directions. Calculated diffuse scattering patterns from the model are in good agreement with observed neutron scattering data. Nanoscale domain formation is driven by the need for the Pb atoms to satisfy their valence requirements; within a planar domain, the Pb atoms are displaced in a concerted fashion away from the centre of their 12-fold coordination polyhedra with an in-plane displacement along <100> towards one of the coordinating O atoms. The B-site cations Zn and Nb display a strong tendency to alternate in the <100> directions but complete order is frustrated by the 2:1 stoichiometry. No diffraction evidence has been found that this B-site ordering is directly linked to the nanoscale polar domain ordering. Such a linkage cannot be completely ruled out but, if it does exist, its effect on the diffraction pattern must be quite subtle. The effect of applying an external electric field has been modelled, and the results are found to be consistent with experiment.