Went to the party. It was, in fairness, a bit hap-hazard at first, tunes were playing while a groups of drummers stood not far away and banged their pots out of time. Then the music stopped altogether for a while and it was just drummers, although they did seem to find some rhythm by this point.
There was even a proper toilet, in one of the doorways off the tunnel, but when we went to use it the second time, the cops were escorting people out and shutting the door. At this point, with about 3 policevans at the entrance and another visible on the bridge at the other end, we decided to pop to the pub, grab some food and use their toilets.
Went back out for a smoke about 30 minutes later and there were no coppers to be seen. When we left the pub, we went for another look and the drummers had shut up, the sound system was a bit closer to the main entrance and there was a great atmosphere. When they played 'Killing in the Name Of', all hell let loose with people surfing the 'crowd' by the soundsystem.
Really enjoyed it and hope to see more events in the future. Uploading a video of the Rage Against the Machine moment as I type - if youtube doesn't cancel the upload for some unknown reason again.