Tax is usually deducted automatically from source including your employer, pension provider and bank savings. These tax deductions on behalf of the state at source are often made under certain assumptions which may not be accurate in line with your tax band or status.
Self Assessment is a system which HM Revenue and Customs (HMRC) uses to ensure tax payable is line with one’s tax status and normally applies to collect Income Tax from people who are not formally employed or have other alternative sources of income.
People and businesses with other income must report it in a tax return which is must be done annually.
The return covers a period of 12 months ending 5th April. If you need to send one, you fill it in after the end of the tax year (5 April) it applies to. One needs to keep records (eg bank statements or receipts) to enable him to compile the required information.