ERCSC flux densities at 545 GHz compared with the colour-corrected 500 μm flux densities of the brightest H-ATLAS sources inside the Planck beam (squares). The dashed line indicates the S_ATLAS = S_ERCSC identity. The filled circles show the colour-corrected summed flux densities, weighted with a Gaussian beam centred on the ERCSC position and with FWHM = 4.47 arcmin (Planck Collaboration 2011b). Only the ERCSC sources with at least one Herschel counterpart with (not colour-corrected) flux density S_{H − ATLAS} > 0.25 Jy are shown in this plot. The outlier with ERCSC flux density  ~ 3.5 Jy is the edge-on spiral galaxy NGC 5746. The outlier with ERCSC flux density  ~ 1.5 Jy is the G12H29 source to be discussed in Sect. 5.