"Due to the complexities of the voting system"

This Telegraph report on the end of the "Brown bounce" caught my attention, especially the lines

"A ComRes survey for The Independent puts the Conservatives up two points on 39 per cent and Labour on 34 per cent, down two, with the Liberal Democrats on 16 per cent, down one point.

It confirms the findings of a YouGov poll for the Daily Telegraph at the weekend, which put the gap between the two main parties at seven points, and also showed a fall in support for the Government over the last month.

Due to the complexities of the voting system, if the ComRes poll was duplicated at a General Election, Labour would remain the largest party but 30 seats short of an overall majority."

The UK electoral system is inherently biased in favour of the Labour party; Labour voting constituencies tend to be smaller than Conservative ones, so a Labour vote is worth more than a Conservative one.