allocated a code = 3 (AC3). ) It should be noted that the training sample is sufficiently low redshift (and also validation sets which are discussed in this paper) that it is safe to assume no evolution with redshift in the quantities of interest.

While each of the galaxies in AC1 consists of an HI source with a single optical counterpart, the relatively large diameter of the Arecibo beam (3.3$\times$3.8 arcminutes) means that more than one galaxy may contribute to the 21 cm flux of a given source (Haynes et al. 2011; Jones et al. 2015), leading to overestimates of the HI fluxes of some galaxies. We therefore undertake a three-step process to remove from our AC1 sample galaxies whose HI flux may be contaminated by neighbouring galaxies within the beam.

First, for every galaxy in our initial training set, we search for all known spectroscopic companions (from Simard et al. 2011) which may contribute significant additional flux to a given galaxy’s HI measurement. Given the paucity of gas in red sequence galaxies, we consider only companions which are blue. Following Patton et al. (2011), we classify galaxies as blue if they have Simard et al. (2011) global rest-frame colours 