Each year all self-employed individuals will be issued with an annual Self Assessment Return shortly after 5 April. For 2018/19, there is a statutory requirement to complete and submit the Return to HMRC before either 31 October 2018 (if you send back the paper return) or by the 31 January 2019 (if you file online).
If you file a paper return after 31 October 2018, you cannot then file another return online to avoid the late filing penalty. If you do not comply with these requirements, you will be fined an initial penalty of £100 and a further £100 penalty at the following 31 July if the return still remains outstanding.