Lord Ashcroft has declared on the Conservative Home website that:

"Not only did no pollster anticipate it, nor did any forecaster, academic, commentator, pundit, journalist or party insider. I believe the highest public prediction — made by Harry Cole, the Sage of Guido — was for 297 Conservative seats."

In factAndrew Gimson, in his "Election Diary" for Standpoint's April issue, not only predicted a Tory majority but got the number spot on:

...a sweepstake was organised on the question of how many seats the Tories would end up with. Flown with insolence and wine, and because I expect the Tory campaign to be more formidable than Labour’s and also because I reckon that when people look at the Tory record over the last five years they will conclude that it merits a second term, I shouted “330” before anyone else had a chance to corner the high end of the market.

No one else wanted to corner it. The smart money was on 50 or 60 seats below that.

The published figure of 331 Conservative MPs includes the speaker, John Bercow; if we exclude him, we arrive at exactly 330.