Call out this afternoon. Hall lights not working. Every time the guy switched them on at the fuse box the breaker flashed and tripped off.
This was the cause. The wire had been resting against the back of the light fitting, which itself contained a 50W GU10 bulb. Those things get hot!
The cable was in a proper rock hard crispy state.
This is why I hate incandescent GU10 bulbs.