Having posted a question about this on the London Overground website, I've just received this reply.....
Dear Mr Sawers
Thank you for contacting London Overground.
I am sorry it has taken longer than expected to repair the cash machine at Forest Hill station. The works to carry out the repair has now been authorised and work will commence in the near future, unfortunately I am unable to provide specific dates.
Please accept my apologies for any inconvenience caused. If I can be of any further assistance please do not hesitate to contact me.
Kind regards
Abdi Guhad
Customer Services Team
London Overground