The Parliamentary Ombudsman can carry out independent investigations into complaints about government departments - and you will be complaining about the Tax Credit Office, which is part of HMRC.
The Parliamentary Ombudsman website used to have some good information about how to complain, but now it seems designed to confuse and say nothing meaningful, rather than to help anyone.
Prepare your complaint as in previous Dispute Steps, using your Case History and any dispute letters or forms. Then write a covering letter to your MP, requesting that they refer your case to the Parliamentary Ombudsman.
The only way the Ombudsman will agree to consider your complaint is if it is referred to them by your MP. Your MP cannot refuse to do this. What they do have a choice over is whether they simply pass the case on, or whether they in some way support your complaint. (i.e. write with the referral 'please review this as unfair' etc.). So, if your MP does not forward it, advise them that you will make a complaint against them.
This stage is only over if and when the Parliamentary Ombudsman write to inform you that they have "decided not to uphold your complaint". Before moving on to the next stage, respond to them complaining if any issues have been left unaddressed or unacknowledged, or you feel that the decision is unreasonable.
Once again, if HMRC hassle you or affect your payments, complain that you are still in dispute and, as per their own legislation, they should not be seeking to recover until this process has been exhausted. You can use our sample "Dispute Status" complaint letter as a guide - just edit it to suit your circumstances. If they start immediately hassling you for recovery before you are ready to respond, you can send them our "Pause Recovery" request letter.
Still not got a fair outcome? See if you can finally get some justice with legal action in Dispute Step 7.