Which certificate for proving payment of NI when working remotely for foreign company?

I am about to start working for a German company remotely from the UK. I am a UK citizen. I will be paying German income tax but not German social security and instead I will be paying National Insurance in the UK. My employer has asked me to provide a certificate proving that I will be paying this. After an initial search online I though that the form (CA8421) - Apply for a certificate to confirm you pay UK National Insurance when working in 2 or more countries - would be the correct form, however I won’t be working in 2 countries. I will just be working in 1 country (the UK) but for a German employer. Can you advise which form I will need to use for my circumstances? Thanks.

Hi, 
You will require a liability decision from HMRC.
If there are no UK operations registered for the company, you may be required to register for self-assessment or pay as you earn for national insurance only depending on the decision.
Please write to:
PT Operations North East England
HM Revenue and Customs,
BX9 1AN
We require the following details,
Full name
National Insurance Number
Date of birth
Date you left the UK – (if applicable)
Your most recent UK address
Your address abroad – (if applicable)
The type of work you do
The company name and address – UK and abroad
Where your contract is held
How often you work in the UK
Your normal working pattern (the days, weeks or months you regularly work)
Thank you.