  We present the results of a multi-wavelength study of the 19 most significant
sub-mm sources detected in the SCUBA 8-mJy survey. As described in Scott et al.
(2001), this survey covers ~260 arcmin^2 using the sub-millimetre camera SCUBA,
to a limiting source detection limit S(850um) ~ 8 mJy. One advantage of this
relatively bright flux-density limit is that accurate astrometric positions are
potentially achievable for every source using existing radio and/or mm-wave
interferometers. However, an associated advantage is that SED-based redshift
constraints should be more powerful than in fainter sub-mm surveys. Here we
therefore exploit the parallel SCUBA 450um data, in combination with existing
radio and ISO data at longer and shorter wavelengths to set constraints on the
redshift of each source. We also analyse new and existing optical and
near-infrared imaging of our SCUBA survey fields to select potential
identifications consistent with these constraints. Our derived SED-based
redshift constraints, and the lack of statistically significant associations
with even moderately bright galaxies allow us to conclude that all 19 sources
lie at z > 1, and at least half of them apparently lie at z > 2.
