Headcode is Victoria to North Kent Line via Nunhead, so would presume somewhere on the LC&DR. It is a Saturday so the train is strengthened by two extra coaches.

Alan Goodwin says "I believe this is an up train approaching New Eltham - the head code is correct for the Dartford Loop.
It looks like 13 on disc. If so the train is possibly the 13.10 Ramsgate to Victoria". - it is indeed duty 13.
This is another diversion this particular weekend due to the Shortlands - Swanley MAS/revised track layout commissioning. Note that the section from Lewisham to Strood is former SER rather than LCDR.