I remember the first one, although I was just a spectator by virtue of a) being dreadfully unfit and having 20 gears to play with. I was amazed that anyone could make it up Highgate Hill on a fixie and the speeds attained on the way back down to Swiss Cottage were impressive...even suicidal in some cases.
It looked like a laugh, so I'm going to build meself a singlespeed/fixie for the winter. Target: complete the course by the end of the year without blowing up or slamming it into a bus stop.
Its mainly the guys on London Fixed Gear and Single speed www.londonfgss.com. There's a thread on there where most of the regulars post to ensure that they're not going to be billy no-mates, as I don't think its quorate every week.