  We report the redshift of an unlensed, highly obscured submillimetre galaxy
(SMG), HS1700.850.1, the brightest SMG (S850um =19.1 mJy) detected in the
JCMT/SCUBA-2 Baryonic Structure Survey, based on the detection of its CO line
emission. Using the IRAM PdBI-WIDEX with 3.6GHz band width, we serendipitously
detect an emission line at 150.6 GHz. From a search over 14.5 GHz in the 3-mm
and 2-mm atmospheric windows, we confirm the identification of this line as
CO(5-4) at z = 2.816, meaning that it does not reside in the z~2.30
proto-cluster in this field. Measurement of the 870um source size (<0.85") from
the Sub-Millimeter Array (SMA) confirms a compact emission in a S870um
=14.5mJy, LIR~10^13 Lsun component, suggesting an Eddington-limited starburst.
We use the double-peaked CO line profile measurements along with the SMA size
constraints to study the gas dynamics of a HyLIRG, estimating the gas and
dynamical masses of HS1700.850.1. While HS1700.850.1 is one of the most extreme
galaxies known in the Universe, we find that it occupies a relative void in the
Lyman-Break Galaxy distribution in this field. Comparison with other extreme
objects at similar epochs (HyLIRG Quasars), and cosmological simulations,
suggests such an anti-bias of bright SMGs could be relatively common, with the
brightest SMGs rarely occupying the most overdense regions at z=2-4.
