If anyone is interested I've just been informed that the departure of my 1900 JL44 LHR-HND flight in December has been brought forward by 75 mins (1745-1650+1), and is now being shown as a 14hr, rather than a 12hr flight.
This makes the connection to JL51 HND-SYD (1725-0620) 95 minutes, which I assume is allowable as my booking still stands.
I just hope there's time for a shower in the F lounge!