Here's something interesting. I grabbed the last 6 months' worth of national opinion polls from Wellsy's and graphed the Tory lead in percentage points. On the tiny chart below, you'll observe that the mean is 10 points; the hatched area shows one standard deviation each side of the mean, and I've plotted a linear trend through it. (You can see a full-size version of it here.)
The interesting bit; there are 21 polls, out of 158, that showed a Conservative lead of more than one standard deviation greater than the mean. All of them occurred before the 29th of January. There are 24 that showed a lead more than one standard deviation less than the mean. 20 out of 24 occurred since the 19th of February. What on earth could have happened between these dates?
The posters broke in a big way around the 19th of January and the second wave hit in early February. Clifford Singer deserves a knighthood for this.