As diligent students of British politics understand, most votes under our first past the post system make no real difference to the result of a general election. The ones that count are those cast by swing voters in marginal constituencies. The Conservatives have been confident in their assertions that, despite a narrowing gap in nationwide opinion surveys, their lead among these defining voters in the marginals is big enough to make the crucial difference. A carefully targeted poll in this morning's Times suggests their confidence may be misplaced.