It's straightforward to use the HMRC tax portal and the "Add missing investment income" option. I've recently updated this (once all my dividends for this tax year were known). They amended my tax code within about 4 weeks, with Xmas in between. So I will pay the correct amount of tax this year without any adjustments being carried over. I stopped having to do self-assessment when I retired, and was glad to see the back of it,