Hi Jan,
Check your policy, I think you might find that it excludes external cleaning, I'm a Ltd company so by law I have to have employers liability for ladder's, its very very expensive! public liability is really cheap, this could open a can of worms, without giving too much away, their are cheaper ways of getting ladder insurance if your a Limited company.
But if your doing newbuilds you and/or your staff do need to be insured for external cleaning.
Regards
Paul