I do loads of builders cleans.
To get off the glass use a new scraper blade, wet the window well, scrape upwards only once then wipe scraper clean the re-wet area and do again.
Most builders cleans have tape on the frames and sills.
It should still be there as removal of this is usually when the builders clean is done.
Pick at a corner to lift it and pull.
All cement and silicon gone in one go.
If there is no tape on the frames and sills use a plastic de-icer scraper wet cement and push down in the centre of it and it should break up.
Do not push it up or down on the frames as it will scratch the UPVC.
The silicon you should just be able to pull that off slowly but if that does not work you need a silicon remover.
This is the condition of the windows are when i get them
Also builder and property developers are terrible at paying.
I am always 4 or 5 months behind in payments.
Paul