A curious red ring, about an inch across, has appeared on my chest. It looks like ringworm but does not itch like ringworm - which I experienced as a child.
I may have found an answer fairly swiftly - a condition called annular erythrema centrifugum. This site says it can be caused by drugs or food "such as blue cheese or tomatoes." Well it's not tomatoes but it might be stilton cheese. Or it might be montelukast. It doesn't seem anything to worry about although the pictures are a little scary.