Soot is generaly acidic in nature and has therefore probably discoloured the upvc. Normally if you can clean it with a high alkaline cleaner within 48 hrs you'll be ok. Assuming this is not the case and the windows are white upvc - try some normal white car t-cut in a small area, if this works, go for it. It can be time consuming but cheaper than replacing the windows, so best to price it per hour instead of piece work, based on your trial area. I've got some other ideas if this doesn't work, but it is the least aggresive to start with.
Best of luck
Chris Netherton