I took out Public Liability yesterday, and in the process had a bit of a chat with the broker.
Regarding glass, it seems the ACTUAL pane of glass you're working on is not covered for the time you're working on it.
However, once you've cleaned it, lets say a piece of your kit comes into contact with it and breaks it then that'd be covered by insurance.
I use a ladder stand-off on 100% of my upstairs windows, there isnt one I dont use it on, I could imagine me putting that through a pane of glass perhaps, but I doubt Id break one with my rubber.