If you are one of the 840,000 taxpayers who missed the 31 January deadline for filing their 2015/16 self-assessment tax return online, you will receive a penalty of £100 for late filing. HMRC has introduced a new online service for making appeals against penalties for late submission of self-assessment tax returns. You can only appeal once you have actually submitted your tax return, unless you can show that you did not meet the criteria for filing a return. In that case HMRC should agree to withdraw the return and cancel any late filing penalties without the need for an appeal.
The link for appealing is here https://www.gov.uk/government/publications/self-assessment-appeal-against-penalties-for-late-filing-and-late-payment-sa370