I tweeted @UKCAT, asking if their research showed that scores vary over the testing period. They said:
"Hi - average scores tend to get higher and then lower over the testing period."
So that's interesting! I've been watching this poll like a hawk, the main effect I've seen is that 726-750 alone accounted for 36% of the scores a week ago, and now things are starting to spread out. The actual "mean", as far as I can calculate it, hasn't changed in that time.