Well, I'm going for the 20th, not checked anything, but as it's a birthday party for a friend, i really don't want to miss it (iirc, it cost me £220 for flights last time)
I went to it the year before last and it was a blast, although starting a weekend underground off with a party is one hell of a mistake.
I might just go for the party really lightweight, and make it an explore the weekend after if you like. I've been meaning to do a few off the track things for a while but haven't really been in a position to do so, I was talking with fb about doing that, but a big 28 meet where we can split off into smallish groups and disappear to meet up again on the evening for giggles sounds like the perfect way to spend a long weekend. I *think* the most i've had under there is about 25-30, would be awesome to beat that.