Roughly how much of a nation-wide margin does Tories need to win the election? It's quite much isn't it?
To win an overall majority of one it's something like 8pts (might be 9... not entirely sure)...
To give an example of how hard it is for the Tories to win a majority if Labour hasn't collapsed, in 1992 the Tories won a tiny majority but lead the PV by a wide margin. If very few votes had switched in a handful of constituencies, Labour could have won and been behind in the PV by something like 7pts...
Reason for this is increased regional polarisation, something that began with Harold Wilson's narrow win in '64 (when Labour failed to win a load of traditional margins in the South, but won previously safe Tory seats in the North), and the fact that turnout in middle class areas is (nowadays) always higher than more working class areas.
The results look distorted and to a certain extent they are, but not as much as it looks so to speak.