On the off chance, and after trying many products, I tried a Magic Eraser from the Flash household section and it was absolutely amazing! A quick dip in a bucket of water to activate the soapy stuff inside and it removed all the baked on black stuff with no effort at all (I've sweated over my tips often enough). Quick wash off and polish/wax and you're good to go. The active 'ingredient', if you like, is melamine foam which is a microscopic abrasive so don't go wailing on the bird poop on your paintwork with it.
OT it looks like it could be a great glass cleaner but I'll test it with a microscope and a glass slide before and after to check it's OK first.
Robert