Hi Paul Weller,
You cannot carry back gift aid relief to an earlier tax year under any circumstances.
If there is an error in the figures quoted in the tax return, eg. 21/22, you have 2 years from the due date of the tax return, to amend your tax return.
E.G. 21/22 tax return due date is 31/01/23 and 2 years from that date is 31/01/25.
If the 2 years have passed, then you can submit an overpayment relief claim (OPR) in writing to HMRC.
Guidance on how to claim OPR can be found at
SACM12150 - Overpayment relief: Form of claims
OPR also has a time limit, which is 5 April 2 years after the amendment window ended.
For 21/22 this would be 05/04/26.
Once the OPR time limit has elapsed, it is too late claim anything.
Thank you.