Hi
Article 17 of the UK / Germany tax treaty covers state pensions:
Germany: tax treaties
When the UK/German tax convention was amended in 2010 it gave the German tax authorities the right to tax social security, what is in effect state pensions in Germany.
As a result the German tax authorities are exercising their right to recover tax that was not previously paid by UK resident individuals on their German state pensions.
The pension should not be declared in a Self Assessment tax return, as it is not taxable in the UK.
You should amend any tax returns 22/23 and 23/24 that include the Germans pension and submit an overpayment relief claim for 20/21 and 21/22 tax years.
For the 2 earlier years 19/20 and 20/21, you will need to make a claim under the MAP procedures as described in article 26 of the treaty.
Thank you.