Aviva Annuity & 1257L Tax Code for Non-Resident?

Hello, Aviva is paying me an annuity and has told me it's paying tax on this as a single person allowance 1257L. The annual annuity is way less than the threshold when personal income tax is charged in the UK. I live overseas and have done for 30 years with no UK income at all. How do I check or inform HMRC about my tax status to ensure I don't have to pay tax? Aviva tells me I need to write to HMRC and if they agree no tax is required HMRC will inform Aviva. If so, where do I write to and anything specific I need to share with HMRC? Thank you.

Hi ThaiGordo,
Please go to:
www.gov.uk and search for 'DT individual'.
Some countries have a version dedicated to them, so look through the versions to find the appropriate version.
Download and print of the form.
Complete it to include all of your UK private pensions / annuities and state pensions, sign and date it and then post it to your local tax office in your country of residence.
The tax authorities there, will validate the form and return it to you.
Please then send the validated for to HMRC at the address on the front page of the form.
Please note that repayments can only be made into a UK bank account.
Thank you.