If you have received income which has not already been subject to tax or reported to HMRC in some form then the likelihood is that you would be expected to register for self-assessment and complete a tax return on an annual basis to report this income to HMRC. This could be in the form of Self Employed income, Partnership income, income from a Limited company, and in the following circumstances;
Rental Income from a property
If you had income from abroad
Your taxable income was more than £100,000
If you were employed and have employment-related expenses more than £2,500.