It could be a fox or it could be a muntjac deer.
I can hear the laughter already at the idea of deer in Forest Hill but this species is about the size of a domestic dog but probably with thin legs http://www.bds.org.uk/muntjac.html

Muntjac deer are very shy and make a barking noise. They can spread along railway lines and have been spotted in Sydenham Woods and evidence has been found around Forest Hill.

If you do think it could be a muntjac deer making an appearance please let me know. I have a planning application to object to that would benefit from some recent evidence of their existence around this area.