A single-crystal cerous magnesium nitrate (CMN) paramagnetic salt thermometer has been set up and used for therometry between 0.4 K and 3.1 K in a cryostat also equipped for the measurement of the vapour pressures of ³He and ⁴He. Two temperature scales, T_{X 1} and T_{X 2}, have been derived in experiments with samples of CMN obtained from different sources. They are in agreement to ± 0.2 mK with the NPL-75 gas thermometry scale above 2.6 K, with the T_XISU (1975) magnetic thermometry scale down to 1.3 K and with each other down to 1 K. There are appreciable differences between the two scales below 1 K for an unidentified reason. The scales are retained in terms of calibrations of 5 rhodium-iron resistance thermometers which also carry NPL-75. Vapour pressure data for ³He and ⁴He obtained with these thermometers have contributed to the derivation of values for the differences between EPT-76 and the established vapour pressure scales, T₅₈ and T₆₂, and also are intended for use in the complete revision of T₅₈ and T₆₂ which is envisaged by the Comité Consultatif de Thermometrie. The relationships between these various scales and other thermometric data in the range 0.5 K to 5.2 K are discussed.