I believe I underpaid tax on two SA returns before 2021. I wrote to HMRC by registered post 12 months ago offering the underpayment and asking for the entries to be raised on my SA account. I have had no response. I chase every 30 days by web-chat, to be told HMRC has my letter but there is a backlog and I can expect a reply within the next 30 days. When each 30 day period expires without a response, I chase again. I expect to pay late payment interest until the time I notified HMRC of the problem, but would HMRC try to charge interest for the 12 months my payment offer has been in the backlog queue?