It is quite fiddly to get into Diagnostics mode. I checked the Face ID which passed but curiously it reckoned that the IoS was not up to date - Apparently I need to move to 26.4.1 - I have Auto Updates "on" but that update wasn't picked up. Thanks for the advice! It has just finished updating and rebooted but I am still being asked to "Move iPhone a little lower". So maybe the camera needs repair - all is crystal clean.