Well, for the UK you can go here....
http://www.caa.co.uk/docs/33/CAP371.PDF
Section B page 12. Paragraph 18 et seq.,
The operator would possible need reminding that if there is another delay down the line then the flight might be grounded ex crew hours, but I do not in fact see what it cannot be done. The eventuality that leads to the extension of the FDP in this case would be used for what actually did happen - delay in FADN after leg 2. There seems nothing about departing home base here. If that is so-then you could arrive for the beginning of four sectors, be delayed due de ice(?) for the leg one departure and then use discretion to complete the full four sectors. One has to remember that the rest period of minimum twelve hours might mean some footwork on the part of Crewing for the next crew duty cycle though?
I think that the crux of this is that you may not use discretion to fit the flight schedule into but you may use it to get home or to complete a disrupted cycle for unavooidable circumstances?