I've also done the safety courses and my take on it is really simple- "ladders can be used for short durations if safe to do so" It is very rare to be up a ladder for more than a few minutes when cleaning windows, so if the surface is secure, the ladder is maintained and you keep your three points of contact, you should be ok.
When I did the IPAF license, the instructor said that a short duration is around twenty minutes. However, if the worker came down the ladder onto the ground and then climbed back up the ladder, that is a new job. So you could theoretically, work for hours up a ladder without breaking the working at height reg's as long as you came down every twenty minutes.
Also, if you were working on a job and it was wfp, but there were a few windows where for whatever reason, wfp couldn't be used, a ladder could be used if more practical.
The guys on here who have said that the homeowners can be sued if a wc fell off a ladder while working on their house are incorrect. It is the wc who does the risk assessment, and the wc who decides what method to use. The homeowner could only be sued if they provided a faulty ladder for the wc to use.