Social Security contributions: Germans in UK

Hello, where can I find succinct information about the situation re social security contributions for German Expats in the UK:
- How the contributions to be paid by and service received by the employee relates to German levels
- How does it affect your overall pension (i.e. German pension and what you get from UK), when you stay in the UK for, say, 3 years
- What do you need to do to guarantee you get back into the German system, when you return. Do you need to pay some contributions to reserve this right - particularly, if you are "freiwillig versichert"?
- Is it possibel to stay in the German system (at least for health) and have them pay your bills in the UK just as if you're on holiday?

I am interested in quality succinct sources as well as your personal experience get a rough idea.

I know: many questions on a boring topic, but I'm grateful for any help.

kidn regards
Sven