London Underground - access to blocked off tube tunnel?

I saw a documentary a few weeks ago--the title was either The London Underground or Underground London; it showed many of the now-hidden underground places. You could probably find it online.

Perhaps they could get in through accessing the London Post Office underground railway system, and crossing through at Waterloo South or thereabouts?

http://en.wikipedia.org/wiki/London_Post_Office_Railway

Do they perhaps gain access when a German bomb explodes and blasts a hole down from above? Or is the Tube system too deep for that? I know from riding it that there are many levels, but as a native, you'd know more about that than I.

Some of the unused tunnels are now being leased out to entrepreneurs. I think around Clapham they're farming in the deep tunnels.
http://growing-underground.com/
http://www.standard.co.uk/news/lond...empty-tunnels-near-clapham-north-9095157.html

Maybe London Underground could have opened up access to the Camberwell tunnels so it is easier for your guys to get in.

People opened up the tunnel during the Cuban Missile Crisis and the secret entrance is known only to a few.

Blogs are your friend.

http://www.placehacking.co.uk/tag/silent-uk/

The photo books DERELICT LONDON and LONDON'S LOST RIVERS are excellent writing inspiration.

This programme Ghosts on the Underground might give you some ideas:
http://www.youtube.com/watch?v=V3gqlm4HTSc
It's a bit long to watch the whole thing maybe. But it's also quite freaky so you might get sucked in...

You'll find plenty of information on disused tunnels, stations etc in London on these two webpages:
http://www.underground-history.co.uk/front.php
http://www.abandonedstations.org.uk/
Be aware that clicking through those sites can make hours disappear though...

I am an urban explorer, although I don't do many tunnels because well I'm a girl, and tunnels are a little too risky in terms of unsavory characters.

The railway tunnel I've been in had storm drain access. They also probably have ventilation shafts large enough for people.

You're in luck - I did research for this one for my own WIP - the vampires operate a small 'haven' thing in the Camden Deep Shelter. Now there's several ways to do this...

1/ Dead Platforms. Surprisingly common, particular in the pre-war 'deep' Tube. You know those mysterious doors you see in the tunnels? Some of them are linking to these dead platforms, and then to... scary things. Think of it - only a half-inch of steel protecting the city's commuters from the evil!

2/ Dead Stations. Not all the stations built ended up being used. A few of the entrance buildings survived; often they'd simply brick up the underground parts and sell off the ticket-hall as a shop or something. Perhaps they forgot about doing the same for the escape tunnel?

3/ WWII bric-a-brac. Everyone knows that the Tube was used as a bomb shelter during the Blitz. But they did more. Whole tunnels were used for government records storage, barracks and command centres. Naturally, all these had multiple exits. Perhaps that odd little building in a park surrounded by trees or a rusted door in the lower basement of an old building is a long-forgotten exit?

4/ Maintenance shafts. Yep, you've picked up on this one. A cliche, but with good reason: there's tons of 'em.

5/ Cold War bric-a-brac. Like the ones from WWII, but newer. There's 'special' tunnels criss-crossing Westminster, along with bunkers, storage rooms etc. Rumours of a special 'escape tunnel' linking Downing St, Parliament, the MoD and out to Heathrow continue to be denied. Perhaps they re-activated the old tunnel, kitted it out as a bomb shelter and put in an entrance in whatever you like?

I'd say this: just make it up. Yes, the particular X might not exist in real life, but if X existed elsewhere, you can simply add another one where you need it to be. As long as you keep it in 'the realms of believability', that is.