Do I need to tell HM Revenue and Customs?
Yes. If this is your first self employment, you will need to complete form CWF1, ideally as soon as you know your starting date. This registers you as self employed and enables you to pay your self employed Class 2 National Insurance contributions. These contributions are payable at the rate of £2.85 per week (for 2017/18) and is usually paid with your Self Assessment tax. If you delay registering, you may have to pay a penalty.