I have been told that Brasso is very good for removing stains from sinks. However do test it out somewhere inconspicuous first! Alternatively there is some stuff that plumbers use which is very effective, but I don't know what it is called. Good luck.
if i have learnt any thing from "how clean is your house" it is this - salt and vinegar will remove anything ANYTHING i tell you...
or see http://www.diydoctor.org.uk/projects/bathrepairs.htm for all your bath and sink repair needs (apparently)