I have my pensions and a small income from another sauce and at the end of the tax year I declare it on 'my page' on the HMRC website and my tax code is sorted for the next year. Very easy to do.
It’s really easy. I’ve been filling in the self assessment form for years.
You can read all the bumph for yourself or if you think it might be complicated, just pay for a one off meeting with a tax accountant who will explain it all. Then you can take it from there.
Having said that my source of extra income was fairly straightforward, from sales of books.