my experience of public liabilty insurance leaves me very sceptical of insurances companys period, Ive been with several over the years who it turns out incredibly dont insure you for the property your working on !!!!
Don't take my word for the following and do check with your insurer. I believe that the bit I've quoted from your post is a common misunderstanding.
The word "property" in this case refers to the window being worked upon. This is because "property" can mean the building where you are working or it can refer to "a piece of property" e.g. like someone's hat or coat or, in this case, someone's window. In insurance jargon, I believe that the latter meaning applies.
As I saay, chewck this with the insurer if you are uncomfortable about it.
It wouldn't surprise me if insurers intentionally confuse the issue with this so that businesses pay more to take out a higher grade insurance policy.
Personally, I do pay for covering the "property" being worked upon. It costs me extra but some of those large industrial panels would be expensive to replace. As there is more likelihood of making a claim, the excess tends to be higher.
I would appreciate some input from others in case I've misunderstood the issues.