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?