My conservatory glass roof has lead staining from the lead flashing immediately above. I have tried many cleaning products and managed to remove some of it with Ciff kitchen cleaner.
A friend then suggested Cillit Bang. I couldn't believe after leaving it on the stain for just 5 minutes the stains just disappeared. I can't promise it doesn't also remove the self-cleaning layer on my glass but to see the stain disappear is amazing.
I am truly impressed but anyone trying this product should proceed carefully and maybe try it on an area that doesn't show. For me it is a final answer to this problem but you must use it in accordance with the manufacturer's instructions and wear appropriate protective eye wear and gloves.