Ladders are legal for short duration work at height, which includes window cleaning, upto a height of 6m provided some sort of stabilisation is use which can be just the rubber feet on the ladder.
Also, all you have to decide before hand is if ladders are the best way of doing the job and have a means to do the job with another method. So carry a set of extendable poles with you and at the job, decide which is best for the job being done.
That basically, is the law now.
As for who is going to enforce it, who knows? Rather like the law about using a mobile phone while driving? I still see mobiles being used while driving just as sometimes I see some of the most idiotic things being done window cleaning.
Cheers