Phew!

I found the wiring fault and it was not behind any of the drywall panels which would have made it extremely difficult to find in a non-destructive way.

It seemed that when the drywall sheet went on over the ceiling fan fitting, the live wire was dragged through the hole, stripped of its wirenut and some of its insulation, and shorted to the grounded metal mounting.

I was lucky to find it, so I put it right, but not all the lights came back. It seemed that the short had fried the dimmer switch, but I found a discarded non-dimmer, wired it in and it all came back to life! I was pleased

Here was the problem:

The left wire, now taped, was trapped against the earthed metal box. Fortunately, it was easy to find and repair.