I have 8 place settings of the Sarah Conran dishes. I have had them for about 2 years and I am noticing that they have black marks at the bottom of the cereal bowls and on some of the dinner plates. I took them in to the Bay where I purchased them and they said it was caused by my stainless cutlery. They recommended I buy Pfaltzgraff stoneware and porcelain cleaner. I can't find it anywhere.Has anyone else had a similar problem? I have never had this happen to any of my dishes before.