HMRC will be conducting maintenance on the self assessment online service overnight on 6 November which means the service will be down for three hours from 10pm to 1am on Saturday morning
Due to scheduled IT maintenance, agents and individual taxpayers will be unable to access the online service between 10pm on Friday 6 November and 1am on Saturday 7 November. If you use commercial software you will be unable to submit your returns at this time.
Details of HMRC service availability and planned downtime for self assessment is available here