I think they have an umbilical attachment to sections of Class War and the anarchist scatterings around London. And I say that as a lefty.
I won't be able to go into all too much detail here as this might turn out to be very outing indeed - but as a formerly active member of the so-called revolutionary left and as someone who still, to some degree, identifies as an anarchist and retains some contacts in the ... erm ... milieu, I can basically confirm that this is the case.
Sadly, and this was one of the major reasons why I eventually withdrew from hard-left activism and joined the International Brigades of Armchair Revolutionaries (IBAR), this is not particularly unusual for far left groups.
In my experience, they are made up of a mix of a) idealistic intellectuals, b) true believers and c) action seekers. Typically, you'll see some overlap between a) and b) and between b) and c) and c) will tend to start hopelessly outnumbering both a) and b) as the group grows.
My personal reason for leaving was not dissimilar to the current situation, though the context was different: I could not personally stand for nor was I okay with being part of a group that was refusing to condemn the Palestinian faction Hamas - not because I was not down with supporting the idea of Palestinian self-determination in principle but because I refused to enter unholy alliances with people opposed to virtually every other belief I personally happen to hold (and I don't mean this in the religious sense).
This is a bit of a tangent, but I thought I'd share in order to maybe give a bit more of a very superficial understanding of typical dynamics to those posters who have never been part of the revolutionary left.