http://www.nationalrail.co.uk/service_disruptions/83516.aspx

---quote---
Emergency engineering work at Lewisham is causing disruption to services
through the station. The crossing that allows trains to run from the
Bexleyheath line through Lewisham towards London Victoria has broken.
This crossing is a specialist piece of equipment and it will take at
least four weeks to build a replacement.

Until then, trains between London Victoria and Dartford via Bexleyheath
will be diverted via Sidcup and will stop additionally at Crayford,
Hither Green and Sidcup.

Passengers travelling between London Victoria and stations between
Lewisham and Dartford via Bexleyheath will need to change trains at
Lewisham. Alternatively, passengers may use stations on the lines
through Woolwich or Sidcup at no additional cost.
[SNIP]
---/quote---


The errant crossing appears to be this one...
http://commons.wikimedia.org/wiki/File:Lewisham_station_MMB_02.jpg
....the broken link from the Bexleyheath line to the route to Victoria
(via Nunhead) is the one running diagonally bottom right to top left.