I made Seville Marmalade today following Delia's recipe but using sweet oranges with, as per your advice last year, adding a lemon for every two oranges. The combo works well. Today, at the fast boiling stage, the liquid was foaming almost to the top of my preserving pan. As I use a sugar thermometer, the foam kept obscuring the reading. I had to keep adjusting the gas flame to manage the foaming and as a result, it took almost an hour for it to reach setting point. This hasn't happened before and I am wondering if you have ever come across this problem or have any idea what caused the excessive foaming.