HMRC have updated their website to confirm that everyone within Self-Assessment is eligible to defer their second payment on account due on 31 July 2020, not just the self-employed. The deferment is optional and you do not need to apply for it.
The payment will then become due on 31 January 2021 along with your 2019/20 balancing payment and your 1st payment on account for 2020/21.
You may notice if you receive a statement from HMRC, that the June 2020 self-assessment statements show 31 January 2021 as the due date for paying the July 2020 payment on account in light of this.
This is because HMRC have updated its IT systems to prevent taxpayers incurring late payment interest on any July 2020 payment on account paid between 1 August 2020 and 31 January 2021. The deferment has not been applied for all taxpayers by HMRC and it remains optional.