I have a question about self assessment tax/interest

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Is your question legally related at all?

We do not answer tax related queries unless they are legally related.

Please set it out and I will see if I can help.

A regular company can't - most of them would be time barred.

However, HMRC is not a regular company, you could try to challenge them by raising a time bar but you would need a tax advisor to asses their response. Have you seen the following: 

https://www.gov.uk/government/publications/rates-and-allowances-hmrc-interest-rates-for-late-and-early-payments/rates-and-allowances-hmrc-interest-rates#:~:text=There%20are%20two%20rates%3A,as%20the%20'minimum%20floor')