In reply to:
In such a scenario, I would argue that PlusNet are at fault as well as being responsible.
Define Plusnet's fault in this issue? How exactly are they at fault, if they admitted the problem exists, why it exists, and assured the customer that their service will not be affected in any way? If anything, in this instance Plusnet have done what they could to reassure the customer that there's nothing to worry about.
They are in some respects responsible for their contractors, and have fulfilled their responsibility to their customer by doing what would be expected of any company in this case.
It would seem to me that you (many of you) are now attempting to blame Plusnet for the smallest blip in service, and add it to a list. Why not. It's what you're good at.