Emerging technologies require increasingly unusual functional materials and so understanding the origins of complex magnetic and electronic behaviour, and how these can be tuned, is increasingly important. The perovskite structure is an ongoing focus of materials research because it can accommodate a huge range of elements to give tailored physical properties. Our proposal focuses on double perovskites, containing both light and heavy magnetic ions. These materials are challenging to prepare, but our property measurements indicate somewhat surprising physical properties. Our proposed neutron powder diffraction experiments will help us to understand the origins of these properties, and help us to design and prepare new functional materials with optimised properties.