The correct sequence, with regard to that bill, should have been:
(1) OR invoice your ISP.
(2) Your ISP invoices you.
(3) You reject the ISP invoice, with your details why.
(4) ISP reject the OR invoice, passing on the information you have provided.
(5) OR and ISP squabble.
(6) ISP comes back to you.
(7) You restate your case.
(8 ) ISP reiterates "No" to OR.
(9) OR checks the logs.
(10) OR apologises to your ISP.
(11) ISP apologises to you (on OR's behalf).
Legally, you are the client of your ISP (not OR) and your ISP is the client of OR. OR should only have done what its client, your ISP, asked it to do. It was in that communication channel that things went wrong.
Of course, if your ISP is BT Retail there
may have been a bit of "
trying it on".
A personal comment. I do not trust BT Retail or BT Wholesale to get things right.
However I cannot fault the OR engineering staff -- having previously spent time talking to members of the current incarnation and also BT & GPO engineers (the previous variants).