MS 0451.6-0305 is a rich galaxy cluster whose strong lensing is particularly prominent at submm wavelengths. We combine new Submillimetre Common-User Bolometer Array (SCUBA)-2 data with imaging from Herschel Spectral and Photometric Imaging Receiver (SPIRE) and PACS and Hubble Space Telescope in order to try to understand the nature of the sources being lensed. In the region of the 'giant submm arc', we uncover seven multiply imaged galaxies (up from the previously known four), of which six are found to be at a redshift of z similar to 2.9, and possibly constitute an interacting system. Using a novel forward-modelling approach, we are able to simultaneously deblend and fit spectral energy distributions to the individual galaxies that contribute to the giant submm arc, constraining their dust temperatures, far-infrared luminosities, and star formation rates (SFRs). The submm arc first identified by SCUBA can now be seen to be composed of at least five distinct sources, four of these within a galaxy group at z similar to 2.9. Only a handful of lensed galaxy groups at this redshift are expected on the sky, and thus this is a unique opportunity for studying such systems in detail. The total unlensed luminosity for this galaxy group is (3.1 +/- 0.3) x 10(12) L-circle dot, which gives an unlensed SFR of (450 +/- 50) M-circle dot yr(-1). This finding suggests that submm source multiplicity, due to physically associated groupings as opposed to chance alignment, extends to fainter flux densities than previously discovered. Many of these systems may also host optical companions undetected in the submm, as is the case here.