I find the best way to keep track of that kind of thing is following a bunch of well connect geeks on twitter. Frustratingly it clashed with both productcamp and the facebook hackathon.
It would be great to have a london events calender both to keep track of these events and also to avoid clashes.
I didn't know about the hack day, but I found this page through Matthew's twitter:
Now really annoyed (mutters to self under breath .. muttter mutter mutter ...)