Delayed Flight Departure Time
Thanks to everyone for their constructive inputs. As it happens, the advice from Dogwatch 55 hit the spot exactly - the departure time on FlightAware is shown as 1920 BST, or 6+ hours late, which matches my recollection pretty closely. (As an aside, I'm still not clear as to how 'departure' time is officially defined - is it doors closed, engine start, push back, take off, or something else?).
Now all I need is a way of persuading BA to comply with the EU regulations about compensation - their response so far has been to say that I'm not entitled to any because the flight was not actually cancelled. Seems like a grey area.