  We observed the starburst galaxy M82 in 850$\mu$m polarised light with the
POL-2 polarimeter on the James Clerk Maxwell Telescope (JCMT). We interpret our
observed polarisation geometry as tracing a two-component magnetic field: a
poloidal component aligned with the galactic 'superwind', extending to a height
$\sim 350$ pc above and below the central bar; and a spiral-arm-aligned, or
possibly toroidal, component in the plane of the galaxy, which dominates the
850$\mu$m polarised light distribution at galactocentric radii $\gtrsim 2$ kpc.
Comparison of our results with recent HAWC+ measurements of the field in the
dust entrained by the M82 superwind suggests that the superwind breaks out from
the central starburst at $\sim 350$ pc above the plane of the galaxy.
