Ohh this thread is not good, made me quite homesick. I'm an ex employee and proper miss some of the real geeks that work on the underground, Mike Ashworth and the heritage people, accessability, timetables, and engineers, happy geeks in their geekdom, I miss them all. Waves at City and Rachel, I was non operational but was privilaged to know many on the front line, heros (both 7/7 and everyday). Anyway as I worked there some time ago some things may have changed.
There's a stream running through Earls Court, you can lift a cover in one of the walkways under the district line tunnels (which are above the picc line) and see the water flowing.
There's a not very secret back exit at West Ken, I think there was an entrance into the flats at Clapham South.
I've walked in the tunnels at night and during engineering works, also entering by South Kentish Town. It was hot, hard to get around because it's not maintained and was full of hot sweaty track workers so not very exciting. Other disused areas can be great fun though, Belsize Park has some excellent places if I remember right.
The office above Chalk Farm was a rentable space, used to be leased to Jay Kay's management, is an amazing shape and that fantastic curved window has a view down Chalk Farm Road to Camden and the city below. Awesome, I wanted to live there.