To answer the last question, which we all seem to have overlooked. 
Mobile workers do not need to be formally notified about a period of availability and its duration in advance. It is enough that they know about it (and the foreseeable duration), in advance.
So, if you’ve been there before, and you know (roughly) what to expect, or if a colleague has forwarned you, then that time spent waiting can be counted as a P.O.A.
As regards the other part of your question. If you arrive expecting a long wait, and it doesn’t materialize, then that makes no difference.
first off I would like to say what a good job you have done trying to suss out this W.T.D the people I work for have still got thier heads burried firmly in the sand.BUT!!! does anybody know how these periods of availability(POA’s)affect somebody like myself who delivers chemicals and by law is the person in charge and can’t walk away from the vehicle
Well done Lucy, nicely decifered 
Although I have not been here reading about the WTD much anyone from the old forums will know how much I personally tried to understand this new legislation & I am sorry to say I think it has failed dismally in trying to bring about a shorter working week for most if not all drivers. As a container driver it affects me not one iota, I know that at most tips unloading will be 2 - 4 hours so I just reasaume that everything will take 4 hours to tip & actually driving much more than 40 hours per week is almost impossibble even if I work 6 full days a week. This goes for most drivers I would assume.
Once again I congratulate Lucy for her plain english translation & I guess we should congratulate the EU Government on such a waste of their time to change absolutly nothing but make it more complicated to get those shorter weeks.
Trevor