Thanks for the advice chaps.

Got a call from a very helpful BA lady at Dulles this afternoon who told me they would keep the checkin open until an hour before this flight and suggested due to the light inbound load that the flight would probably go at 00:30 versus the 01:05. Just got a text from BA indeed confirming the 00:30 time.

Restores one's faith in BA when you get a call from such a downright helpful person.