Not sure about depot drivers, they are not trained by mainline driver instructors once they leave classroom based training at Bolton (training school).
So far this year courses have started in Jan, Feb and April. There is one trainee driver course planned for June, at least two other courses planned for later this year last time I checked, possibly Sept & Nov, I can't remember. Based on figures available at the time, the numbers of trainees required for each depot on each course are allocated up to 12 months in advance, and updated at intervals. All trainee numbers for each depot are subject to adjustment depending how the May '14 diagrams affect depot establishments once finalised, and obviously any existing drivers leaving the company in the interim. These diagram changes are the biggest recast since the Dec '08 change.