The upgrade of the network on the cross-London Thameslink network will lead to delays for passengers in coming months. From December, the line between London’s Blackfriars and St Pancras stations will close every night between 22.30 and 04.30 and over most weekends, meaning that passengers will need to use the Tube system, on which tickets will be valid (link below.)
http://www.abtn.co.uk/Network_upgrade_promises_delays_for_airport_passengers

The closure is a precursor to work that begins in earnest next year on upgrading the line between Bedford and Brighton to allow longer trains and increased capacity.