For tax return that can no longer be amended, an overpayment relief claim can be made instead. An OPR claim for 2018 to 2019 must be in HMRC possession no later than 5 April 2023. All tax years 2017 to 2018 and earlier are now out of date and no relief will be accepted.
There is a specific format for claiming this relief. Claims not submitted in the correct format can be rejected. All OPR claims must be in writing and must be made by the person who is due the relief except for overpayment relief claims arising from mistakes in partnership returns.
You can find guidance here:
SACM12045 - Who can claim: Partnerships
You will find the full intructions for making an OPR claim here:
SACM12150 - Overpayment relief: Form of claims