Bintel, and probably others, sell
UHTC cleaner. The guys will give you a rundown on how to use it. The glass is pretty scratch resistant so as long as you use gentle wipes with a clean wet tissue first to remove any dust you are unlikely to do any damage as you polish off the marks. Michael showed me how on my C8 and I was surprised at how much pressure the Celestron approved polishing method uses.
Andrew