Bad news I am afraid...
The following is from Southern Railways:
Southern Railways wrote:
The current position is that the gates are being closed at 8pm. It is not proposed to alter that arrangement whilst Southern remains the station operator. This is not the situation that I outlined previously for which I apologise. This is partly due to internal miscommunication on our part and also to changes to the specification of the gating project. The station transfers to London Overground (LOROL) on 20 September 2009 and they may want to adopt different arrangements.
The arrangements discussed with Southern, the Mayor, local MP and London Travelwatch predate the installation of the new entrance to house the automatic ticket gates on the Perry Vale side of the station and its construction has reduced Southern?s options for managing access to and from the station. Southern has been undertaking two gating programmes. One for Transport for London ahead of the transfer of stations (including Forest Hill) associated with the East London Line extension project and another for the Department for Transport. In the case of the latter, Southern has, where appropriate, been installing remote supervision and closure/opening controls on gateline entrances. This has been done to allow subsidiary entrances to remain open when the entrance is unstaffed. These arrangements have not been installed on gatelines/entrances for stations transferring to LOROL as this was not part of TfL?s specification. Specifically at Forest Hill a further complication arises because there is no secondary gate alongside the gateline building. This was originally considered by Southern when designing the project but the gate was not thought necessary by TfL. It appears that the difference in specifications for the two gating programmes was not fully appreciated across the business which led to some incorrect assumptions about how the entrance could / would be operated. Essentially, commitments were given about opening times based on the assumption that the gateline could be remotely supervised when this is not the case. The situation has not been helped as I understand that no notices were displayed at the station to say when the entrance would be open. This has been raised with the station manager and notices should now be in place.