NIC invoice for non-citizen, concerning 1990???

[tunneldude]

Hello,

yesterday I received an invoice from HM Revenue&Customs, stating that my National Insurance Contributions Bill where due for payment now (~900£).
I am a German and since my stay in the UK is now going to be 20 years ago, I'm really puzzled with this.
Also it's the very first time I'm told to be indebted with NIC.

The only explanation provided is “arrears owned prior to 11/11/09”. So all I can do is take a guess as to what it might be about.

I registered self-employed in 1985, earning less than 30£/week on average. This self-employment was terminated in 1987 and an official notice was written to the local Social Security Office.
After that I had been employee until I finally left the UK in 1990.

Is it really possible arrears arose out of those employments?
Is there sure an entitlement of the invoice they send me, especially even though they came up with it a little ...delayed?
May be National Insurance intends to transfer my partial state pension entitlement to Germany, but if so I had not been asked. I'd rather not pay and loose those tiny state pension bits. The bill doesn't look like it's debatable.
If you had some advice for me on how to handle this I'd much appreciate that.
Thank you.