Herschel PACS 100 µm image of HD 1461, as an example of the Herschel data used in this paper (Sect. 2.2.2). North is up, and east is left. From left to right, the plots show the observation, observation after PSF subtraction, best-fitting disc model, and residuals after model subtraction. Contours show ±2 and ±3 times the root-mean-square intensity. Whilst the disc is resolved, it clearly has a lower resolution than the example ALMA image in Fig. 2. Our Herschel disc models are axisymmetric; the offset between the disc model and the origin reflects the imperfect Herschel pointing rather than a physical offset between the disc and the star.