Along with about 250 other people, today I attended a meeting organised by the British Consulate, several topics were discussed, among them SGK. The consulate conceded that in the early days, their understanding was that everybody who had been resident for one year or more was required to register with SGK, whether you intended to join the scheme or not.
He then went on to say that following negotiations early last year that allowed UK citizens to decide whether to join the scheme or not, the rules were further clarified and it seems that if you have registered with SGK, it is likely you are part of the scheme and may well owe a fine plus back payments from to the date you registered.
He then went on to say that the only way to come off the scheme is to either, surrender your residency and leave Turkey, become a citizen,, marry a Turkish National or Die ( the last one seems the easiest )
He did warn that once unpaid back payments reached a certain level, it was likely that the authorities would pursue you and this would include computerised notifications to customs officers at the borders.
He also clarified the rules where some had reported being charged large amounts of back payments when first joining the scheme, he went on to explain that you should join the SGK within a month of the first anniversary from obtaining residency, if you fail to do this and leave it for example 18 months, you could be charged six months back payment, I use the word 'could' as he pointed out that sometimes the clerk processing you may take a more lenient approach.