Been watching this reported on Twitter and London Traffic feeds, sounds very serious.

@MPSLewisham 52s
We are dealing with a road traffic collision there are road closures in place South Circular shut between Sydenham Hill and Dartmouth Road

A205 London Road South Circular Road Forest Hill, both ways between Sydenham Hill and A2216 Dartmouth Road
A205 London - A205 London Road (South Circular Road) in Forest Hill closed and queueing traffic in both directions between the Sydenham Hill junction and Forest Hill Station, because of a serious accident. Diversion in operation - for bus routes 176, 185, 197, 356 and P4. Police directing traffic. Congestion to the B218 Brockley Rise westbound junction and to The Grove.