I use a really good paste from ettore. I cant remember what it is called though but you simply get a damp cloth put some paste on a rub it in to the glass and then rinse off. Its kind of like a cutting compound.
So struggled like hell and tried everything. After posting this was about to have the chat with the customer about not being able to get it off etc. Then just whacked a load of glass polishing paste with my flex polishing machine. Set it to fast and literally cut the hell out of it.
Very messy but it came up beautifully . The glass polishing paste didn’t work with a cloth in a small section I had to be very aggressive with my machine