We investigate the region around the Planck-detected z = 3.26
gravitationally lensed galaxy HATLAS J114637.9-001132 (hereinafter
HATLAS12-00) using both archival Herschel data from the H-ATLAS survey
and using submm data obtained with both LABOCA and SCUBA2. The lensed
source is found to be surrounded by a strong overdensity of both
Herschel-SPIRE sources and submm sources. We detect 17 bright
(S870 > ˜7 mJy) sources at >4σ closer than
5 arcmin to the lensed object at 850/870 μm. 10 of these sources have
good cross-identifications with objects detected by Herschel-SPIRE which
have redder colours than other sources in the field, with 350 μm flux
>250 μm flux, suggesting that they lie at high redshift.
Submillimeter Array (SMA) observations localise one of these companions
to ˜1 arcsec, allowing unambiguous cross identification with a 3.6
and 4.5 μm Spitzer source. The optical/near-IR spectral energy
distribution of this source is measured by further observations and
found to be consistent with z > 2, but incompatible with lower
redshifts. We conclude that this system may be a galaxy
cluster/protocluster or larger scale structure that contains a number of
galaxies undergoing starbursts at the same time.