  [Abridged] Aims: This work focuses on one lensed system, HATLAS
J142935.3-002836 (H1429-0028), selected in the Herschel-ATLAS field. Gathering
a rich, multi-wavelength dataset, we aim to confirm the lensing hypothesis and
model the background source's morphology and dynamics, as well as to provide a
full physical characterisation. Methods: Multi-wavelength high-resolution data
is utilised to assess the nature of the system. A lensing-analysis algorithm
which simultaneously fits different wavebands is adopted to characterise the
lens. The background galaxy dynamical information is studied by reconstructing
the 3-D source-plane of the ALMA CO(J:4-3) transition. Near-IR imaging from HST
and Keck-AO allows to constrain rest-frame optical photometry independently for
the foreground and background systems. Physical parameters (such as stellar and
dust masses) are estimated via modelling of the spectral energy distribution
taking into account source blending, foreground obscuration, and differential
magnification. Results: The system comprises a foreground edge-on disk galaxy
(at z_sp=0.218) with an almost complete Einstein ring around it. The background
source (at z_sp=1.027) is magnified by a factor of ~8-10 depending on
wavelength. It is comprised of two components and a tens of kpc long tidal tail
resembling the Antennae merger. As a whole, the system is a massive stellar
system (1.32[-0.41,+0.63] x1E11 Mo) forming stars at a rate of 394+-90 Mo/yr,
and has a significant gas reservoir M_ISM = 4.6+-1.7 x1E10 Mo. Its depletion
time due to star formation alone is thus expected to be
tau_SF=M_ISM/SFR=117+-51 Myr. The dynamical mass of one of the components is
estimated to be 5.8+-1.7 x1E10 Mo, and, together with the photometric total
mass estimate, it implies that H1429-0028 is a major merger system
(1:2.8[-1.5,+1.8]).
