I usually look at the stain and shake my head and say nothing will move that except a specialist chemical, I can order it in and try, but be aware whatever has caused the stain it will happen again, and the chemical will be around £20 plus my time ect, at this point they say just leave it, just thought it would take a minute with some magic formula you made up
but if they they say yes whatever it costs just do it, then I will try get an answer, so far it hasn't happened yet.