Not dark but black! The flash wasn't attached to the camera. I emphasise that a visible preview went black immediately upon setting the camera to manual. On Sunday morning, when I first posted, there was plenty of light but preview was black apart from the shooting information. It was NOT subject failure.
Thanks for showing me the video. It related to flash on camera, not studio flash. Shooting in RAW, which we were, doesn't need a change to WB or colour temperature. All we wanted to do was to read the aperture requirement, set that on the camera with an appropriate shutter speed and get shooting.
None of our lenses showed any difference.
Did either of you pick up that the solution was to switch focusing from M to either C or S? Now what's that all about?