Southern Railways have confirmed that they intend to start running 10 carriage trains through Forest Hill from this December.
I doubt all trains will be 10 carriages, but at least some will be during the peak period (hopefully most). Before this happens Network Raill need to do some minor platform lengthening, so we should be seeing work beginning soon.

This could increase peak capacity on our line by up to 14% (taking account of Southern and ELL trains). The downside is that when work begins at London Bridge to accomodate Thameslink (from 2013) we are likely to see a temporary reduction in the number of trains going to LB via FH.