We have a 16 year-old loo (purpose built building). The loo seat had gone noticeably brown. We have few visitors and when we do we just brazen it out. However, it had reached the point where I was thinking of replacing the seat or painting it. Drastic measures. Just had a go now with bleach on a scouring pad and it came up just fine.
I hope this post is of use to someone, somewhere.