I know this is a late suggestion. I've only just come to the forums for the first time since just after AdventureX last year!
And mOds - I can't believe you didn't give me the chance to be the first to get a ticket for the second year in a row!
Anyway, you may want to speak to some of the guys on the Games Culture course at London South Bank University (LSBU) and/or Gamecamp. The campus is in Elephant and Castle which is in central London and near a number of overland train stations and is on the Northern line (for Londoners like me) so it's dead easy to get to.
They have held the GameCamp event a coupe times at the Keyworth building. For those that don't know, it's an (un)conference (
http://gamecamp.org.uk/). I went to the last one and it had a couple hundred people, so they can cater for that, plus there are a few rooms of different sizes to choose from in the complex (no offence, no sure AdventureX is going to draw quite such a crowd!) to suit your purposes.
They would be very friendly I'm sure to giving advice on venues and running the event if you need it. Hope the advice helps. Keep me posted on what happens.
I think the main thing you need to do this year m0ds is to do more marketing. it doesn't have to be you per se, just get someone else to do if you like. Definitely take advantage of facebook, advertise on websites. Get a press pack to send out to magazine websites to make it easy for them to do a post about AdventureX so you'll get easy publicity (once you've made the pack up and a template email, it's just a case of sending it out to various people).
Look forward to making it again this year!
Cheers
Tom (random dude from last year who came!).
(EDIT: I've also just noticed that I haven't received an email about AdventureX 2012 yet....has one been sent or is there something wrong with the email list? Obviously the first port of call for publicity...)