  We present observations of a remarkable submillimetre-selected galaxy,
SMMJ16359+6612. This distant galaxy lies behind the core of a massive cluster
of galaxies, A2218, and is gravitationally lensed by the foreground cluster
into three discrete images which were identified in deep submillimetre maps of
the cluster core at both 450 and 850micron. Subsequent follow-up using deep
optical and NIR images identify a faint counterpart to each of the 3 images,
with similar red optical--NIR colours and HST morphologies. By exploiting a
detailed mass model for the cluster lens we estimate that the combined images
of this galaxy are magnified by a factor of ~45, implying that this galaxy
would have un-lensed magnitudes K_s=22.9 and I=26.1, and an un-lensed 850micron
flux density of only 0.8mJy. Moreover, the highly constrained lens model
predicted the redshift of SMMJ16359+6612 to be z=2.6+/-0.4. We confirm this
estimate using deep optical and NIR Keck spectroscopy, measuring a redshift of
z=2.516. SMMJ16359+6612 is the faintest submm-selected galaxy so far identified
with a precise redshift. Thanks to the large gravitational magnification of
this source, we identify 3 sub-components in this submm galaxy, which are also
seen in the NIRSPEC data, arguing for either a strong dust (lane) absorption or
a merger. Interestingly, there are 2 other highly-amplified galaxies at almost
identical redshifts in this field (although neither is a strong submm emitter).
The 3 galaxies lie within a ~100kpc region on the background sky, suggesting
this submm galaxy is located in a dense high-redshift group.
