An email received yesterday from Southern;

Engineering works are carried out by Network Rail as they own the railway infrastructure. Much of their work is carried out overnight but where necessary they may take longer ?possession? of the track e.g. over a weekend when a more intensive amount of work can be completed. See their website - http://www.networkrail.co.uk/aspx/761.aspx

Southern must then work round these engineering periods to provide the best possible service to passengers. This will often be a rail replacement service if no rail lines are available or it may involve a diversion to another station. The very best place to check for information is the national rail website which is updated frequently and some weeks in advance http://www.nationalrail.co.uk. Posters will also be displayed at the station.

Currently, the next set of engineering works planned is for Sun 2 Dec when those travelling into London Bridge will be routed via East Croydon but a direct service from London Bridge to Forest Hill will be available. The next weekend when a similar pattern is required is Sunday 3 Feb. More works will occur as the project progresses and passengers are urged to check their route on the nation rail website before they travel.

The main reason behind the works on this line I believe is to do with the works to bring the East London Line extension to the area from 2010. More information about this project is available at: http://www.tfl.gov.uk/corporate/projects.../2105.aspx