Samples of Ln3Ba2Mn2Cu2O12±y, Ln=Sm, Eu, have been synthesized with non-absorbing isotopes and studied by high-resolution powder neutron diffraction. For Ln=Eu, the material was studied before and after annealing in oxygen. Results show that segregation of the manganese and copper between the layer sites is incomplete and that oxygen can also be incorporated between the CuO5 square-based pyramids giving the overall composition Ln3Ba2Mn2Cu2O12.1. Cation size mismatch on the A-type cation sites leads to a local distortion in the oxide sub-lattice in the MnO6 octahedra and consideration of the A-site coordination geometries permits an understanding of the limited lanthanide range for which this structure forms. While the Ln and Ba cations are fully segregated the observed B cation disorder and additional inter-cuprate layer oxygen are likely to preclude observation of superconductivity in this family of materials.