No Reply to HMRC Letter - What now?

I made a mistake on my last Tax Credit information form (forgetting to deduct pension payments from income) and didn't realise until I got a letter saying I had received an overpayment which I would have to pay back. I rang HMRC and after many hours of being on hold spoke to an agent who gave me an address to write to to notify HMRC of my error so they could recalculate. He couldn't do it because I had been transferred from Tax Credits to Universal Credit so he couldn't make an amendment to the account. I sent a letter to the address he gave me at the end of May but haven't received any acknowledgement or response. HMRC has since written to my husband and I to say they will take back what is owed through UC or PAYE. We haven't claimed UC and don't have PAYE as we are both self-employed so I'm not sure what will happen next. I urgently need somebody at HMRC to read my letter and amend my error. How do I do this?

Hi HMRCForumUser,
You will to call the Tax Credit helpline to check if a record of the letter/call is held and the query can be taken forward from there.
Tax credits: general enquiries
Thank you.