New manganese-based double perovskites Mn2-xCaxFeReO6 have been synthesised at high pressure. Mn2FeReO6 has spin-active cations at all sites; S = 5/2 Mn2+, S = 5/2 Fe3+ and S = 1 Re5+ and has TC = 520 K. Very high magnetisations are observed up to M = 5.0 BM/f.u. for Mn2FeReO6 (the previous record in double perovskite ferrimagnets was 4.3 BM/f.u.) Mn2FeReO6 also has novel MR properties. 5 days of WISH beamtime are requested;- 2 days for zero field studies of x = 0.5 and 1.5 between 10 and 300 K, to determine crystal and magnetic structures. -3 days for high field experiments on Mn2FeReO6 to explore the three spin ordering regimes discovered in initial neutron experiments. 50 mg polycrystalline samples will be used. The Mn2FeReO6 sample will be pelletised to minimise field reorientation of grains. Results will be essential to help understand these new spintronic materials.