Taxpayers with payments on account due 31 July 2020 have already been allowed to delay payment until January.
The Chancellor has announced that there will now be a concession for payments due in January 2021, allowing later payment.
HMRC anyway runs a helpline to agree ‘time to pay’ arrangements. It may be that the new concession will mean payment can be delayed without the need for any formal agreement, possibly without interest being charged. I shall update on this as further details are announced.