Storm Glands.

AJJewsbury: 
 

It was a split load consumer unit. The circuits to the right were not R.C.D. protected in the old board. 

Indeed - and the faulty circuit was for sockets - so more than likely on the RCD side even on a “16th Ed” board. A large G/Y had also been snagged onto the capping, so providing a reasonably low earth fault path. I still think it seems implausible that the fault was pre-existing but didn't give any problems for the householder.

    - Andy.

You are building a strong case Andy that the video producing sparks was responsible for the cable pulling and damage, or his side kick who has now left.

Z.

AJJewsbury: 
 

It was a split load consumer unit. The circuits to the right were not R.C.D. protected in the old board. 

Indeed - and the faulty circuit was for sockets - so more than likely on the RCD side even on a “16th Ed” board. A large G/Y had also been snagged onto the capping, so providing a reasonably low earth fault path. I still think it seems implausible that the fault was pre-existing but didn't give any problems for the householder.

    - Andy.

You are building a strong case Andy that the video producing sparks was responsible for the cable pulling and damage, or his side kick who has now left.

Z.