Spoke to BMW dealer today and guess what......
No talk of potholes or kerbs but an agreement to supply a new rear wheel at their cost!
A local motor sport wheel specialist told me he thinks that the BMW wheel problems are caused by the loads which are not absorbed as well by the run-flats as conventional tyres due to the extremely stiff side walls.
I asked whether BMW would honour their warranty if I replaced the run-flats with ordinary low profile tyres. They said that as the run-flats have a strengthened tyre wall the handling characteristics using a conventional tyre would be different and there might be difficulties with the warranty if a claim was made and the tyres were not run flats.
I have to say that the dealer's service manager has been helpful throughout.
It seems as though BMW are getting to grips with the problem but I am not sure that replacing the existing wheel and tyre set-up with exactly the same set-up is sensible. The dealer is obviously following instructions from BMW HQ.
I thing I will drop an email to their technical people to try and get some sense as to why this is happening. Will keep this forum informed.