My HMRC account is linked to an old phone number which is not active anymore. I have tried to reset the number online as well as using the HMRC chat agent and the phone support, however as I am not able to answer one of the security questions I am not able to pass the security check. The security question that I am not able to answer is which month and year I initially registered for self-assessment and unfortunately I am not able to retrieve this information from my documents.
I am struggling to get any information on who I could contact now in order to reset my security details, for example whether I can escalate my problem somewhere or whether I need to send a letter to someone within HMRC.
Any advise and help would be highly appreciated.