Frederick Forsyth explained the pointlessness of national polls this morning on Radio 4
If labour wins a constituancy with a majority of 10,000, then they've still only won a single constituancy, but if the tories wins 10 constituancies with a majority of only 200 in each, then they've still beaten labour.
So if the 40% of mori poll respondees who say they're voting labour are concentrated in fewer constituancies than those voting against labour, labour could be in trouble