Get proper cover, its cost more but its worth it. I have Federation Of Window Cleaners public liablity. It covers the glass Im actually cleaning, chemical damage, key loss, cherry pickers, ladders, wfp, everything really and up to an infinite height!!
Oh and it has tool cover too! I clean some VERY expensive glass, like huge ancient leaded windows and also 18th centuary ricoco mirrors that are worth thousands upon thousands. Most cover wouldnt include the stuff your actually working in! But imagine I had a bill for a 100,000 pound window!