Thanks to all those who replied to my question, and pointed me to other members of the forum who have asked similar questions. I spoke to the dealer today, and they admitted there was a problem with rust showing through the alloys. They told me that although it wasn't covered by warranty, they would remove all the wheels, mask off the discs etc, and paint the affected areas with ultra high temperature resistant silver paint for £40. I'm not going to do this for a while, because I managed to remove the existing deposits with some fine sand paper, and a wire brush. However, over the course of the winter, I'm sure the rust will get worse. I didn't see much point getting them to do the job now, only to find that rust also came through on areas that hadn't been painted. So, I'm going to sit tight and let the elements do their worst, and then get it sorted. £40 seems pretty cheap considering the amount of work involved, and I can't help thinking this may be in recognition of the lack of proper treatment the wheel hubs and components receive when they leave the factory. Still think they should cover this under warranty though, because the rust looks awful!