Good luck PJS, I have the same rims also, but removed them from the car to fit the M6 reps. It would be worrying me greatly if I still had them fitted as my car is about the same age as the ones failing.
BMW MUST see the light on this one - it is far too much of a co-incidence that these rear wheels are cracking. Chances of hitting a pot hole with both rear wheels and not damaging a front must be very slim??? Surely BMW can see this??