My understanding on block times was that the schedule was in place seasonally. If on the day the predicted block time was longer the FDP was based on the scheduled time rather than the actual. This was of course on the assumption that the block time was accurate (which with so much time spent on OTP these days they normally are, apart from Ryanair who seem always to be much longer). The flight however was a one off. There must be data though on the flight to/from as these are standard sectors the bit in-between not.
In the days of CAP371 this was a constant discussion on long haul flights e.g. the old up to 7 and factorisation of ETOPS flights. You'd have thought the Captain would have run into that before and had similar discussions. In this case if the schedule was 8hrs 55hrs but due to headwinds 9hrs 40mins the FDP was based on the schedule times not predicted.
Or am I barking up the wrong tree?