"I have contacted our technical team and they have explained Plusnet have blocked our website and therefore NS&I customers who use Plusnet as their internet provider are unable to access our website"
As a Plusnet customer I have continued to have access to the NS&I website during the reported outage. Therefore I must conclude that there is an incomplete understanding of the underlying problem. Or an inaccurate public report of what the problem was.
"Will we ever know what this was all about? I doubt it!"
Possibly NDAs are part of the obfuscation.