I also received a registered letter from the SGK,and had it verbally translated,the gist of which is as follows.
To meet the requirements to join the SGK you must provide documentary proof that you are retired,ie receiving a state pension,also proof you are out of the NHS.
These two documents must be translated and stamped by the notary
Further more,if you have been in the SGK,have received medical treatment,operations etc,and cannot provide the documents required,your insurance payments will be returned and you will have to pay for any treatments.
You have 30 days to comply with the requirements.
My translator did not mention any payments being returned if you had not made a claim.