As far as your comment about BT informing you that you didn't need to tell your ISP... BT may well own Plusnet but there are chinese walls all over BT due to regulatory compliance. BT Retail will have no connection to Plusnet other than the fact they share a parent company. They are not allowed to deal with Plusnet any differently to any other ISP.
The fact BT Retail gave you bad advice is not the fault of Plusnet - so if you have an issue with that it should be BT Retail you take it up with. Plusnet have admitted a failing in their systems. They have (I think) admitted that your case was then not handled properly. They are trying to rectify the issue.
If you still aren't satisfied then I would suggest you need to write to Plusnet and tell them what would satisfy you (whether that is a written apology, financial compensation, etc). If they then refuse to do that then you either take them to ombudsman/court or you write it off as a bad experience.
What you have managed to do is highlight an issue in their systems that hopefully they will rectify and you can have the (possible) satisfaction of knowing that others hopefully won't go through what you did - it's down to you as to whether that gives you any satisfaction of helping your fellow man/woman.
Thaks for your comments. PN did not pull the plug on me because BT may have given me daft advice when my number was changed. It is as clear as water that they did not check validity and activity properly when my old number returned No Service Attached.
Issues such as short-lived line faults can happen at any time, perhaps to PN's credit I only had a couple such issues in 10 years and downtime was minimal, most internetters would put up patiently being down a few hours without bringing back up systems, and in an imperfect world nobody would kick up a sting for that.
My understanding of a backup system includes dial up facilities and alternative external fast internet (I had both) but on the whole my back up systems enabled me to manage with a big "limp" and it was all very cumbersome and costly in real money terms and personal inconvenience.
I hope this incident has helped PN members to press them for improved automated systems and honing the human element of customer service. They knew they had disconnected an active customer but once they noticed the blunder the CS was robotic and unresponsive to urgent service restoration. I can't accept that my 10 year loyalty is repaid in this manner.