  Hyperfine spectroscopy of positronium formed in the presence of a static
magnetic field is considered. Generalising the situation hitherto developed in
the literature, the magnetic field is not assumed to be parallel to the
momentum of incoming polarised positrons, while the possibility of electron
polarisation is also included in the analysis. The results are of application
to high sensitivity positron polarimeters used in current $\beta$ decay
experiments.
