old, not bold
"..they set off not knowing about their extra 50 tonnes...." Not at all credible old but bold, how could they not know??..We don't just fill her up and launch into the air muttering " oh, I wonder how much fuel is on board"? There's the small matter of the Flight Crew agreeing/accepting the fuel figures with the ground personnel, the EICAS fuel display (flight deck) has to be checked when setting up the fuel configuration up prior to departure and the small matter of the Captain signing for the fuel prior to doors closed....
And I agree with woodpecker, over the years most of us on the Fleet have positioned empty aircraft with various fuel loads and I have never heard of problems with trim. The only way 50 tonnes of fuel and no pax would put you out of trim would be if the refullers had seriously stuffed up, and again the crew would most definitley be aware of that fact before starting engines.
Frankly there has been no mention of this alledged incident in the offices at BA, so either it was management foul up and a lid been kept on it ( and I'm not saying that's impossible but the stories always leak out) or more likely, IMHO, it's a good rumour/story but it didn't happen as has been described and ths thread should be put to bed.