That's what insurance companies are for, taking the P that is.
I never bother with "including the property being worked on" - this means only the actual pane of glass you are cleaning, and they stick massive excess figures on to discourage you from making a claim as well as hiking up the premium.
So long as I am covered in the event of me causing massive damage to a building, or serious injury to a member of the public, I'm happy. I'll take my chances that any "actual property being worked on" is very unlikely to get damaged anyway, and even if I were to break a pane of glass, the cost is likely to be far less than the excess I would have to pay in the event of a claim.