Well this is old news now, but here goes.
Under the Thameslink programme, the 3 bay platforms at Blackfriars have been closed. Therefor Catford Loop trains from Sevenoaks are continuing through London to either Kentish Town, St Albans, Luton or Bedford. Since the Traction Supply changes from 750v DC to 25kV AC at Farringdon all stock for this service must be Dual Voltage - ie, class 319s.
Hence almost all of Southern's 319 fleet has been transfered to FCC for this purpose (remembering of course that 319s are also still used on FCCs Sutton Loop and most Brighton services).
One of the conditions of TL2000 is that crews shouldn't changeover throughout the journey as far as is possible. Consequently FCC have had to learn the (Southeastern) road to Sevenoaks, and SE crews have had to learn the FCC road to Bedford.
'Ownership' of the train changes over at Blackfriars, so Southbound trains belong to SE and northbound trans belong to FCC.
This can give rise to the weird situation whereby you get on a train at Sevenoaks which has Southern livery, is driven by an FCC driver, but is being operated by Southeastern.
You couldn't make it up could you !
(I've heard that some 465s may go over to the Southern temporarily, but this is because they're surplus since they're not needed on the Catford Loop)
HTH
FP