Here's a thought...(I'm assuming the cable in the loft, with no earth, the insulation was so degraded it crumbled in Frosty's hand)
The person who did the CU change could well have carried out a global L&N to E IR test and it passed.
Because the suspect cable has no Earth, the test wouldn't have picked up on it.
So you could argue the CU installer is not at fault.
However, a L to N IR test would possibly have indicated a fault.
Anyway, I hope your burns are not too bad, horrible injuries.
Get well soon!