MnPS₃ is a layered honeycomb lattice antiferromagnet with the spins aligned perpendicular to the layers at low temperature. Structurally it is monoclinic, with an unusual combination of magnetic and crystallographic symmetries, and so offers a rare insight into the behaviour of low-dimensional magnetism. Here the effect of magnetic dilution by Zn substitution on the magnetic structure is explored using neutron diffraction and magnetometry. The composition dependencies of T_N and the spin flop field are found. The critical concentration for long-range order is estimated to be 45% Mn. The first neighbour exchange is found to be independent of composition. It appears that dilution induces a moment component in the plane, and gives rise to a complex phase diagram in which a reordering temperature is apparent.