You can try with the slightly sinister sounding Adjudicator’s Office once you’ve gone through the HMRC official complaints procedure. A friend of mine did this and got monies owed plus interest.
https://www.gov.uk/guidance/how-to-complain-to-the-adjudicators-office-about-hmrc-or-the-voa
So many services are absolutely shit and getting worse. Nothing shocks me any more. NHS, police, HMRC. Is there a single service that actually works as well as it did 20 years ago in this country?