Bastax

Hello, I am a British citizen who has been working overseas for 20 years for different employers. I currently live in Uganda which has a dual tax agreement with the UK. I recently began working here in Uganda for a UK-based organisation who are paying my salary into a UK bank account. I have been de facto non-resident of the UK for 20 years, but am now being taxed on this income. I understand to have an NT code I must prove I am non-resident. The P85 form appears to be for people who are leaving the UK in the tax year, whereas I left many years ago. Can you advise me of the best way to inform HMRC that I am non-resident and that I should so should have an NT code? Thank you.