HMRC's call centre workers are striking on the 31st January.
In a press release HMRC says:
"To make sure our customers are not disadvantaged if they cannot get through to HMRC's call centres on 31 January, we will not impose any late filing penalties for people who file their self-assessment returns on 1 and 2 February."
"The SA deadline remains midnight on 31 January. But HMRC will treat all returns that come in by midnight on 2 February as though they were submitted by 31 January. No customer will have to pay interest on payments due on 31 January that are paid on 1 or 2 February."