Under article 17 of the UK/Germany double tax treaty the pension should not have been taxed at all in the UK. Your pension provider should have applied a "NT" (no tax) PAYE code to account for your non UK/German tax residence status.
The preferred way to rectify this would be to amend any other PAYE codes to refund the overpaid tax on this pension, however that is not an option if you have no other UK PAYE income.
At worst you may have to wait until the end of the tax year i.e. 6 April 2020, and then write to HMRC:
- Enclosing a copy of your P45
- Explaining that you live abroad and have been non-UK resident for 30 years and have no other UK income/gains
- Explaining that the income is not subject to UK tax under article 17 of the DTA
- Asking for a refund by cheque
The address to do this is Self Assessment, HM Revenue and Customs, BX9 1AS, UK.
However, HMRC may be persuaded to give you a refund earlier than at the end of the tax year if you explain these circumstances to them. It seems unfair that would otherwise have to wait until next year to receive your refund which should not have been subject to tax in the first place.
The telephone number to call is +44(###) ###-####and you will need your national insurance number to hand so that they can identify you.
If you have any follow up questions, I am happy to help. Otherwise please consider a 5 star rating for my answer.
Kind regards, Peter