Received Late Submission Penalty but Tax Return form was returned on time

I've received a Late Submission Penalty from HMRC but I did submit my Tax Return form by post on time and have Certificate of Postage and proof of receipt. I've tried every route I can think of to discuss this with them over the past few weeks but have been unable to communicate with anyone due to unanswered calls that are eventually dropped (after about 45 mins) & unanswered emails to hmrcsupport@tax.service.gov.uk. HMRC refuse to provide me with access to digital services such as the Government Gateway and on-line Tax Returns due to my personal circumstances. This compounds the difficulty in contacting them. They continue to demand payment and state that I will incur further charges if I delay payment. I'm at my wits end, completely stressed out and don't know how to proceed. Can anyone offer any advice on how I can resolve this?

Hi,

All our contact details for our Self Assessment team are listed here:

Self Assessment: general enquiries

You will need to provide the reference for the posted item so that we can trace it if it does not show as received on our records.

Thank you.

Hi, I've tried using all of the contact mechanisms provided by this link and others. I don't have Government Gateway access as I'm blocked from using this service - due to my personal circumstances. I have tried many times to get through on the phone but my call is never answered - the call eventually hangs. I have previously used the email address: hmrcsupport@tax.service.gov.uk to get details as I had some problems 2 years ago. I have signed the relevant documents to declare that I'm aware of the risks of using email. I used the same email again and I have provided all the details: the Certificate of Postage reference and the link to the Royal Mail web site showing that the letter had been collected. I did receive an automated reply with a reference number saying that I would receive a reply within 2 working days but that was on 22nd February 2023. I realise that the tax office is under huge pressure at the moment and I just need to be reassured that my query will eventually be answered.