Hmmm... had mine around January 2003. Might have changed since.
First stage was a presentation about the company, question&answer time, finally a test on what has been said, i.e. study the company, fleet, destination etc before you go to the assessment and you'll feel more at ease.
Second stage was a maths test, no calculator, pretty straight forward.
Panel interview was 2:1 in the afternoon, very long (mine was over 1.5 hours), mostly customer service questions and other questions like "where do you see yourself in 2-5 year time?", "why do you want to be a cabin crew?", etc.
I was suprised to be interviewed by 2 staff that had nothing to do with the previous part of the recruitment, though I must admit it seems to be a normal procedure within BA.
Btw, they seem to like people with previous airport/flying experience as half the people who got to the final stage were already in the airline industry.
Good luck.
FJ