do you know anyone with one of those little steam cleaners for cleaning cookers ect. you could try using something like that to try and soften the paint being careful not to put too much heat on the plastic,you dont want to melt it! Emulsion will soften quite easy but gloss will take more work.you could wait til winter when its cold because paint becomes more brittle and will chip off with a scraper unless you've got some dry ice! it will be a slow job but keep at it and the end will come.hot water on a sponge pressed on the paint for a while would be the next best thing.DON'T use heat next to really cold glass in winter, it may crack.hope this helps you.
phil holmes
August 2009