TrogAir
The plan as far as I'm aware is to take the 400 cadets in tranches over the time period.
Seeing as the first are starting in 2012, the last in 2016, this would imply
400 cadets / 4 years = 100 cadets per year.
The schools would probably not want to train much more than this number, and BA will want a staggered and steady increase in pilots to match the arrival of new aircraft. So you're fairly safe waiting until the next tranche.