yep, I have accidentally stumbled into a few sessions at the Half Moon and they were fantastic. I usually had to be up for work at 6am the next morning though and sometimes it did not feel so fantastic any more! LOL.
Used to get a number of members of Oxford folk outfit Magpie Lane there; they are worth checking out and even chatting to as they really knwo the local scene. They were very friendly and obliging. Not sure if they are still going....