Why must I fill in a self-assessment tax form when other people are let off?

Your recent article about more pensioners being pulled into income tax talks about the simple tax calculation many people receive in the post after the end of each tax year. Many people fill in a self-assessment tax return for income earned above the £12,570 personal allowance. More people are breaching the frozen £12,570 income tax threshold - particularly pensioners with small private pensions - but do not necessarily have to fill in tax returns. Heather Rogers, founder of Aston Accountancy and This is Money's tax expert, replies: Your first stop should be HMRC’s online tax return checking tool. When should you fill in a self-assessment tax return?