Framer
My understanding is that if the scheduled block time is realistic for a season and this can be proved by statistical info on block times then if on the day the FDP becomes illegal (say for example aircraft has a limitation or there are headwinds) the the flight can be made.
Prior to EASA the UK CAA had a group of FTL Industry expects which met on a regular basis to give them advice on such matters. It was advice but in most cases it was taken on-board. The body included CAA, BALPA, Airline Reps and some retired guys who probably flew with RAT5 or Douglas Bader.
Now under EASA I'm not really sure what happens in cases of "interpretation" if it is the responsibility of the National Authority or EASA.
My own opinion is as above but in the case of a single one off FDP which caused so much angst - I'm with the Commander.