In many cases [such as the Local/European elections earlyer this year] its not so much about the Tory vote going up as the Labour vote going down. Alot of Labour supporters [my family included] would never dream of voting tory so when they become disillusioned with Labour they stay at home or vote third party.Im not sure if this is the case in this election* but its worth bearing in mind that.
[A]Labour has been held responsible for the crisis as they were in charge. The "speaker of the house" who's supossed to stop this sort of thing was a Labour MP
[B]I think in some ways the expenses scandal was the straw that broke the camels back. Labour has been in power since 1997 so they,ve managed to pick up alot of baggage on the way.
*Edit: The concervative vote only went up by 6.3% whereas the Labour vote went down by 26.7%