If HMRC has asked you to complete a self-assessment return for 2018-2019, you must complete it before 31 January 2020 or face a £100 fine. Some people (those who are self-employed, for example) need to complete a tax return every year; others are sent one because they have untaxed income (often from property).
Although the publisher endeavours to make sure the information in this calculator is accurate and up-to-date, it is only a general guide nothing in it constitutes taxation advice by Which?. You should consult a qualified tax adviser who can consider individual circumstances if taxation advice is required. Which? Limited does not accept any liability for any loss or damage suffered as a consequence of relying on any information contained in this guide.