A group calling itself the Whitechapel Anarchist Group - or WAG - is using the ramshackle building as its 'war den', where tales of violence against the police and capitalist buildings are swapped.
It was here that my infiltration began. During the secretive discussions in the shabby building, it soon emerged that this group was central to the violence and incitement which marred yesterday's protests.
At the first meeting, the leader of the bizarre group of misfits - mostly white working class British men in their twenties - revealed that he was taking orders from Chris Knight, a 66-year-old anthropology lecturer.
With his trademark top hat and coattails, Professor Knight has become known as the public face of G20 Meltdown, the umbrella group which represents more than 100 protest factions.
While his occasional incendiary remarks have ruffled a few feathers - not least his employers' - he is unlikely to pick up a chair and hurl it through a bank window, despite his theatrical public protests.
He is more content to pull strings behind the scenes and let others do his dirty work.
At each of these anarchist meetings, or 'war summits', the leader has stood up to announce the 'latest orders from Chris Knight'.