It wasn't until a really close look behind the scene was undertaken that the reason for the odd light flicker became apparent. Many of the old copper wires were joined with chocky blocks that had green copper ends held in place by the screws. The resistance created by the corrosion had created so much heat at some time in the past, the plastic surrounds of the chocky blocks had melted in some cases.
What to do with the old bookcase hole. More thinking and planning required.