In conclusion, HackDay was brilliant. I met some cool people, had great coding from 10am to midnight, catching up with friends from various places, writing AJAX, presenting my app in front of a few hundred people, and generally having an adventure.
The presenting was cool. I went on second, and then had to go straight away. It was nerve-wracking, but quite a buzz when my hack was voted as useful. I wish I could have stayed to see the others, especially the Hacked game (which I won).