Yup, I got that same letter. This is now the second time while with "BMW" insurance, that they have unashamedly hiked the price unexpectedly. First time they changed my premium, because they had suddenly devised a new rule that introduced a MINIMUM premium, regardless of the value of the car. So my premium went up by R180!! Now a 18% hike! I contacted them, to find out how my premium got hiked 8 months after first insuring a specific car, and they told me they don't have a period of guaranteed premiums, meaning they can hike the premium when they see fit. Other insurers will guarantee your premium for at least a year.
I called DD whom with I have my home/bakkie/bike insurance and they quoted me about R150pm less, but this included a R3500.00 excess as well as only insuring the car for a value of R90k. In fact they struggled to find the right vehicle to evaluate in the Mead & McGrouther guide, also worrying. With BMW Insurance I am insured for a value of R140k and no excess for R604. Difficult to make a true comparison, but at least BMW Insurance seem to recognise the vehicle's value better, which will be important if I ever have to face replacement.
Maybe someone out there knows of an insurer who specialises in less run of the mill models etc. I mean who do the M5 guys have their cars insured with?