Chris88 is right to highlight one of passing comments in London Reconnections of which I was not previously aware - the possible reduction in services on our line into London Bridge.
A reduction in the peak service from 6 to 4 trains per hour (4 to 2 in evening peak) would put additional pressure on Overground services as it would act as a disincentive for passengers from the Northern line to switch to National Rail rather than the (marginally shorter) journey on Jubilee and Overground (ELL).
With only four trains per hour leaving London Bridge to Forest Hill there is less incentive to use these slower trains, despite the extra seats. If you have to wait for 30 minutes for a train then you are simply not going to bother.
I guess I need to find out more about the possible reduced services from December if I can. If anybody else finds out more please post it here.