The main problems with a lot of these laws and directives is they are not worded in a simple straight forward way, unlike other laws, for instance you cannot drive above 70mph on a motorway.
The wahd directive is worded that it can have several meanings to who ever is reading it, for instance you can only use ladders if there is not other practical alternative, so if you read this then on most houses a pole or wfp is suitable, but scaffolding or cherry pickers is not practical.
The other bit about short duration of using a ladder, is 10-15-20 or 30 minutes a short duration, well yes you will all say, but the cleaner is not just going to do that one job in the day, he will move onto the next door and spend another 30 minutes up a ladder, and he can do this all day, which might be for 12 hours, so would you class this as a short duration on ladder use.
At the end of the day if a accident happens, then you will have to prove you are in the right, if you are a sole trader, then its likely going to be the insurance company you will have to convince, if you employ thedn its the HSE you will have to show you used the correct method, and you also have to hope that both of these look at the wahd directives the same way that you do.
Of course coming off ladders and going onto wfp, does not let you free from the other H&S laws that are about, trips and slips can also happen.