The reason they have the "material worked" on bit is to stop claims for scratched glass as a result of fabrication debris picked up on after builders cleans.
Fabrication debris is a result of bad cleaning of the equipment. As its heated up to strengthen, fragments of glass dust stick to the underside. Norman scraping during a builders clean to get the labels off then dislodges the fragments and leaves a nasty "scratch". It appears to be from using a blade, but really fabrication debris is a ticking bomb, it will at some point get dislodged.
The way around it is to get the builder to sign a waiver, he gets one from the manufacturer, stating the normal cleaning methods will be used and that the cleaner is not responsible for the bad work of the glass fabricator.
In short, if you put your pole or ladder through a conservatory you will be covered, even though it is the property you are working on. (I have checked with the underwriters) However, you are not covered for "scratched glass" as a result of "fabrication debris".
Google both terms, it makes interesting reading.
It will also help you see that the higher premium and excess are not really justified as those that include it won't pay out for scratched glass anyway. They will however allow you to pay the £500 excess when you put your ladder/pole through a roof!!