help getting mortar off upvc double glazing and the window

does anyone know how to get mortar off upvc with out it ruining them? the house we moved into has had all the front re-rendered and the builders got mortar over the upvc and didnt think to wipe it off. Ive tried everything in the last 5 weeks even scraping it with my nail which got some off the glass but not the upvc,
the window company said hot soapy water and a scourer but i dont think thats a good idea surely it would scuff the upvc?
any ideas here guy would be great