I've been here a few times and have to say just how cool this place is. It's under LB and is a vast cavernous space. It only opens from Weds - Sat, but if you get a chance I really recommend it.
It's in the downstairs bit of LB and is not recognisable with any name, just a single door leading to the longest walk to the bar area I have ever seen. Still I'd say it's my favourite place to go for somewhere a bit different and great to have in SE1.