Guidance: Self Assessment: service availability and issues.
HMRC provides a dedicated webpage to report the current availability and any technical issues affecting the Self Assessment online service. This information is relevant to all taxpayers and agents who use the online portal to file their returns. You should check this page if you experience difficulties accessing the system or completing your submission.
Source: https://www.gov.uk/government/publications/self-assessment-service-availability-and-issues. Summarised automatically from the official source — general information, not advice.