Hi Mariska Talirz,
You will need to download an print off the form DT individual UK/Germany here:
Double Taxation: treaty relief (Form DT-Individual (Germany))
Please fill out the form, declaring your UK pensions / annuities and send the completed signed form to the German tax authorities.
You will need to find their address.
The German tax authorities, will validate the form and send it back to you.
You should then send the validated form to HMRC.
The HMRC address is on the first page of the form.
HMRC can then refund any overpaid tax into a UK bank account, provided you include your account details or send you a payable order.