I used to quite regularly travel from Hither Green to London Charing Cross on weekends, and on weekdays approx 5-6pm.
On weekdays from about 4pm to 6pm, in addition to the usual services from Orprington/Sevenoaks and Dartford/Gravesend, there were some services which arrived Hither Green as ECS where they entered service, ran fast to London Bridge then Waterloo East and Charing Cross.
These trains were almost completely empty (on average 2 passengers per carriage, although quite frequently a carriage to myself). It was quite clear that these services were run simply to get stock into London for peak out-of-London services, and I assume the reason why they started at Hither Green rather than further back is so they had the flexibility to get stock from Dartford/Gravesend (platform 5) or Grove Park (platforms 1 or 3) as needed. These trains usually arrivied at platform 5, although did sometimes arrive at platforms 1 or 3, despite platform 1 not usually being used for stopping services.
The station destination boards and automated announcements would make an announcement "The next train at platform X is not for public use. Please do not board this train.". The very first time I saw this, I didn't board the train (I thought it probably was the Charing Cross service, but wanted to stay on the safe side), and then my Charing Cross service had vanished from the boards. In future, I always took them, despite the announcements.
Many times there were confused looking people so I would say "This is the Charing Cross service". They would look at me confused and usually tell me that the boards say it is not in service. I would reply "I know, but it is the service to Charing Cross" and then get on. Sometimes they would follow and sometimes they wouldn't, but I did my best to help them.
I was quite impressed that they bothered to pick up passengers rather than run ECS. It was always helpful that I had more frequent services, nicer to have quicker trains and more comfortable not to have to see or hear other passengers.
I presume these still run, but I don't know as I moved out of the area in 2006. I did stay the area for a few weeks in 2009 and they were still running.