I would definitely second Neville's 2nd Thursday singaround, although it invariably manages to fall when I've got too many other things on.

I would also strongly recommend the Woody Guthrie Folk Club, unless you're one of those damned conservatives who don't like radical songs in case they give the servants ideas of course.

A word of caution though. The joint invariably sells out and early. Hence admission is by ticket only, available from We Got Tickets. For more details go to http://www.woodyfolk.co.uk/ . However, the next night is the last Thursday in January, and there are no details on the site as yet.