Hi!
I'm in the exact same situation (British, live in Germany, wife is non-EEA national). At present we're stuck in purgatory waiting for an EEA FP application to be returned but in the past it has always been smooth sailing. I always used the following cover letter successfully (modified to your situation slightly - fill in the blanks).
I don't directly mention Surinder Singh but I do put that our "center of life" is Germany and demonstrate that I'm working here. That should be enough, you can also add that your job here means you are "exercising treaty rights".
I write this letter in support of the application of my wife, xxxx, for an EEA Family Permit. I am a UK citizen, resident in the UK until xxxxx, since when I have been resident in xxxx, Germany. For the duration of my stay in Germany I have been working at xxxxxx, holding the position of xxxxxx.
The initial trip covered by this permit will be a xx day trip to the United Kingdom, <reason for trip>. <Description of trip plans - when to travel, where to stay, what to do etc>.
This will be the first EEA Family Permit for my wife, we chose this route as we travel together to the UK several times per year to visit family and friends. My center of life is in Germany and we fully intend to remain resident in Germany for some years (at least) to come. We live together here in Germany in a rented apartment and are registered as German residents with the German authorities. Our careers here in Germany are both going well and we have firmly established a home for ourselves here. To support this I have enclosed copies of the following to support my center of life being in Germany:
• My UK Passport
• Joint Apartment Rental Contract
• My Employment Contract, Recent Payslips and Annual Salary Statements
• My German Registration Certificate
• My Recent Mobile Phone Bills and Our Joint Landline/Internet Bills
• My German Current Account Statement
• My UK Current Account Statement
• Our Joint Current and Savings Account Statements
If you have any further questions please do not hesitate to contact either my wife or I for more information.