We present recalibrations of the GALFORM semi-analytical model of galaxy formation in a new N-body simulation with the Planck cosmology. The Planck Millennium simulation uses more than 128 billion particles to resolve the matter distribution in a cube of 800 Mpc on a side, which contains more than 77 million dark matter haloes with mass greater than 2.12 × 109h-1M⊙ at the present day. Only minor changes to a very small number of model parameters are required in the recalibration. We present predictions for the atomic hydrogen content (HI) of dark matter halos, which is a key input into the calculation of the HI intensity mapping signal expected from the large-scale structure of the Universe. We find that the HI mass - halo mass relation displays a clear break at the halo mass above which AGN heating suppresses gas cooling, ≈3 × 1011h-1M⊙. Below this halo mass, the HI content of haloes is dominated by the central galaxy; above this mass it is the combined HI content of satellites that prevails. We find that the HI mass - halo mass relation changes little with redshift up to z = 3. The bias of HI sources shows a scale dependence that gets more pronounced with increasing redshift.