Thanks for all the input Guys, and for the welcome. Whatever it is it's been there for some time, Months at least, so I doubt if it will degrade further. I think you may have pegged it blue skies, residue from an attempt to clean off some finger marks perhaps? I will go with the cautious advice and just leave it (as long as I don't notice any serious abberations when viewing) and no doubt in six months I will have forgotton all about it.
