I think some key statements are ..
Captain's discretion may be applied by him in respect of his crew
However...
The crew must agree to the application of discretion
crew were perfectly within their rights in refusing to extend, especially bearing in mind that the flight had not yet left its departure point
Discretion ....unforeseen circumstances occurring down route. It may not be used for the purposes of planning a flight.
Bravo Bravo Bravo for the cabin crew in taking this stance. There is no way that a Captain will enforce his way upon me if it is illegal and I am not in agreement with it..in todays environment of lots of work, maximum FDP's, minimum rest and low low low wages it is unreasonable to expect happy and "willing to go the extra mile" crew members.