Re: NI contributions and catch up
 

Reading the page you've linked to, I think there will be an advantage in buying voluntary contributions.

Let's assume you have a working life of 40 years, with 8 spent in the UK and the rest in Germany:

If you don't pay any more contributions, you will be entitled to a pension from the UK based on 8 years contributions and a pension from Germany based on 32 years contributions.

If you pay Voluntary NI Contributions, you will be able to increase your UK state pension without lowering your German state pension, so you will get more money.

The EU rules on the page you link to only ensure that you are not penalised for not working the whole of your working life in one country. They do not alter the fact that you can increase your UK state pension by making voluntary contributions, without reducing your German state pension.

Re: NI contributions and catch up
 

I agree with this, but you should check with domestic law that the pension isn't reduced if you also get a foreign pension. I have no idea what the rules are in Germany and the UK, but the US does have a law that reduces social security payments if you have another pension that was paid for with wages that didn't have social security taxes withheld, this does not apply for voluntary payments.