The Parliamentary Ombudsman can carry out independent investigations into complaints about government departments - and you will be complaining about HMRC (and possibly even the AO and their handling of your complaint / dispute). The PO investigate that the service being complained about kept to its own policies. In this case that is COP26.
Prepare your complaint (i.e. update your case history / letter with all the recent events making sure to include in the summary what points of COP26 you feel they haven’t met), and then send to your MP and request s/he refers your case to the Parliamentary Ombudsman. Note that your MP cannot refuse to refer your case to the Parliamentary Ombudsman and you do need the MP to be the one to send the initial complaint in.
If HMRC hassle you for repayment or affect your current tax credit payments, complain, using our "Dispute Status" complaint letter; state that you are still in Dispute and as per their own Code of Practice (COP26) they should not be seeking to recover until this process has been exhausted.
Again, it is common to receive a partial success at this stage. The PO may decide that HMRC are responsible for one or more portions of the overpayment and rule that this portion be written off, or HMRC may even admit to it in their response to the PO.
This stage is only over if and when the PO writes to inform you they have “decided not to uphold (all of) your complaint”. If you are still left with an outstanding amount, respond back to the PO complaining if any issues have been left unaddressed / acknowledged and / or you feel the decision is unreasonable. (See our Failed Dispute response template).
Still not got a fair outcome? Then it's time to take a look at your Final Options.