  The SCUBA 8-mJy survey is the largest submm extragalactic mapping survey
undertaken to date, centred on the Lockman Hole and ELAIS N2 regions. Here, we
present new 1.4-GHz imaging of the depth and resolution necessary to reliably
identify radio counterparts for 18 of 30 submm sources. Armed with this greatly
improved positional information, we present and analyse new optical, near-IR
and XMM-Newton X-ray imaging to identify optical/IR host galaxies to half of
the submm-selected sources in those fields. As many as 15% of the submm sources
detected at 1.4 GHz are resolved by the 1.4'' beam and a further 25% have more
than one radio counterpart, suggesting that radio and submm emission arise from
extended starbursts and that interactions are common. We note that less than a
quarter of the submm-selected sample would have been recovered by targeting
optically faint radio sources, underlining the selective nature of such
surveys. At least 60% of the radio-confirmed optical/IR host galaxies appear to
be morphologically distorted; many are composite systems - red galaxies with
relatively blue companions; just over half are found to be very red or
extremely red; contrary to popular belief, most are sufficiently bright to be
tackled with spectrographs on 8-m telescopes. We estimate the median redshift
of the S(850um) ~ 8 mJy submm galaxy population: if the radio/far-IR
correlation holds at high redshift, and our sample is unbiased, we derive a
conservative median z >= 2.0, or >= 2.4 using spectral templates more
representative of known submm galaxies [abridged].
