Hi,
If you have overpaid into the pension, then you need to declare this in a Self Assessment Tax Return, so that the overpaid element can be charged to tax.
You would need to submit SA100 (tax return) and SA101 (additional information) on page Ai4 under pensions savings tax charges, boxes 10 and 11.
You can register for self assessment below, if you have never registered for self asssessment before.
Check how to register for Self Assessment
Thank you.