1640-1800 - customs/immigration
1800-1830 - NRT express to Tokyo
1830-1930 - Keihin Kyuko express (still called that?) to Yokosuka Chuo
1930-2130 - Genji
Keikyu (Keihin Electric Express Railway Company) trains run to Yokosuka Chuo Station.
Keikyu's main station in Tokyo is Shinagawa Station. Note that Keikyu Express
急行 trains do not stop at Yokosuka Chuo, but all others (limited express, local, rapid etc) do.
If possible take a NEX (Narita Express) to Shinagawa Station instead of Tokyo Station. If you take a NEX to Tokyo, transfer to any JR line down to Shinagawa Station and transfer to Keikyu there.
Off topic, Yokosuka Chuo station is breifly mentioned in this music video about Keikyu lol
http://www.youtube.com/watch?v=-JVJZTRpYxg