Intrinsic Gaussian centre and width (σ) of the distribution of Cu-Kα redshifts in each CCD for the observations which went into the stacking analysis, after correcting each observation for the average Cu-Kα value and the time-dependent maps (Fig. 6). This analysis excludes observations with statistical uncertainties on the Cu-Kα redshift of greater than 0.1 per cent. Results for FF and EFF modes are shown separately.