Good Luck PJS
As of today, I am now in the same boat!!
Went to get new back tyres this morning and was told there was a crack in one of the rear 19" 225's.
I then took it to a local alloy repair engineer and he said that it is not pothole damage but stress damage and said that he would not recommend fixing it because of this as it will prob crack again.
He then showed me another alpina 19" wheel which looked perfect and then when you turned it round there were 4 similar cracks all round the wheel.
He also said that he get's loads of bmw wheels in that have similar cracks.
Right - off to phone the dealer - wish he luck
The car now looks funny with an old MV2 18" wheel from my E46 coupe on the back.