Double Taxation Treaty Relief - Without National Insurance Number

Hello Forum, I live in Germany and have worked for a London based company for 3 years now. I sent the Double Taxation Treaty Relief form to HMRC. They say I need a National Insurance Number. I can not get a National Insurance Number because I do not live in the UK. I am currently working and paying Tax in the UK under a temporary national insurance number. I just do not know what to do anymore. The German tax man is asking for all the tax money I have paid in the UK for the last 3 years.

Hi,
In order to get a National Insurance (NI) number abroad, you need to get a liability decision from HMRC to confirm that you are entitled to a UK NI number. To obtain a liability decision, please write to HMRC at the following address:
PT Operations North East England
Insurability Department
HM Revenue and Customs
BX9 1AN
Please provide the following information in your letter:
•    Full name
•    Date of birth
•    Most recent UK & or abroad address
•    Companies UK name
•    Companies abroad name
•    Companies UK address
•    Companies abroad address
•    Where your contract is held
•    The type of work you do
•    How often you work in the UK
•    Your normal working pattern (the days, weeks or months you regularly work)
You must include in your letter that you are asking for a NI number to be issued to you. Once you receive a reply to this request, you will be advised if you can apply for a National Insurance number, and how to do this.
Thank you.