Interesting, in that I took this matter to the Ombudsman and lost.
I complained to Ofcom through the Ombudsman, because BT's own systems stated I was entitled to the discounted line £11.99 rental, when I checked online through their systems, yet I had Broadband from a separate supplier and had done for the whole period. The discrepancy showed that BT were originally going to apply this to all BT landline only customers, then did a last minute deal with Ofcom, to just apply it to those didn't take Broadband from another supplier (which made no sense because VM doesn't even use the same wires into the house).
One of the issues as part of the complaint, was the Ombudsman refused to include Ofcom's actions in the complaint.
The Ombudsman ruled that the amount I was personally out of pocket was 'insignificant' (because the line rental charged remained at £18.99, I left soon after) and on that basis ruled against me, even though replicated across the board (affecting large numbers of customers at once), the amount was very significant, in the millions. It seemed like a stitch up by the regulator at the time or certainly the individual that dealt with both my original complaint, and after escalation, when I disagreed with the decision.
I left BT Retail soon after, moved all business and residential lines to competitors, and BT Retail haven't had a penny from me since.
Complaints to Ofcom/Ombudsman achieve very little, it's an absolute merry-go-round of time wasting where each individual customer has to complain about exactly the same issue affecting millions of customers at once. The Ombudsman make more per case, (circa £200 or more) than the person making the complaint, even if the case is unsucessful.
The system needs a complete overhaul, and not before time. Ofcom/Ombudsman are about as useful as a chocolate teapot, and that's me being polite.