The submillimetre-bright galaxy population is believed to comprise, aside from local galaxies and radio-loud sources, intrinsically active star-forming galaxies, the brightest of which are lensed gravitationally. The latter enable studies at a level of detail beyond that usually possible by the observation facility. Aims. This work focuses on one of these lensed systems, 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 HS T 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 zsp = 0.218) with an almost complete Einstein ring around it. The background source (at zsp = 1.027) is magnified by a factor of μ ∼ 8 − 10 depending on wavelength. It is comprised of two components and a tens of kpc long tidal tail resembling the Antennæ merger. As a whole, the system is a massive stellar system (1.32+0.63 −0.41 × 1011 M ⊙ ) forming stars at a rate of 394 ± 90M ⊙ yr−1, and has a significant gas reservoir MISM = 4.6 ± 1.7 × 1010 M ⊙ . Its depletion time due to star formation alone is thus expected to be τSF = MISM/SFR = 117 ± 51Myr. The dynamical mass of one of the components is estimated to be 5.8 ± 1.7 × 1010 M ⊙ , and, together with the photometric total mass estimate, it implies that H1429−0028 is a major merger system (1:2.8+1.8 −1.5