Slade Green has a few trainees going through at the moment but the depot is pretty much full.
Dartford on the other hand has about 7 or 8 leaving by the end of the year, with trainees already coming through to take their place.
As bionic said they'll re-home the Orpington drivers first (December), it looks to be a bit of a wait for trainees off the street at the moment.
As previously mentioned here, the drivers are unhappy in metroland with many of them putting in for moves to coastal depots...
Charing Cross is having a mass exodus at the moment, I've heard about 15 are leaving up there. (but again any orpy surplus can fill those spaces :-\ )
I'm not sure if they take high speed drivers off the street though? Maybe an avenue of exploration?