The report from NESO is available here if anyone wants to read directly rather than interpretation of journalists -
https://www.neso.energy/publications/north-hyde-review
TLDR as I see it - NESO/NG/SSEN appeared to be unaware of the consequence of failure of one supply on Heathrow (i.e. a total shutdown to manually reconfigure) making their lack of maintenance focus even more stark and Heathrow was running on the assumption that the power supplies were so resilient that a 10-12 hour manual switching operation was sufficient mitigation for loss of a power supply because that'd probably never happen.
The result we saw is what happens when these contrasting positions meet head to head.