I agree that there are an unusual set of factors associated with this election, not least of which being there are two parties to kick for the last 5 years, not one.
But even the uncertainties listed by Gromit are uncertain.
Will the Tories, actually, lose all that many votes to UKIP apart perhaps from in a handful of seats in specific areas.
Will the Lib Dem vote actually collapse? In areas where they are well established I would hazard a guess that they may struggle but not necessarily collapse. And in seats where they have been the main opposition to the Tories are people really going to let the Tory back in by voting UKIP/Labour/Green etc?
The Greens are on 8%+ at the moment in the polls, but if they keep on explaining their policies to peole that figure could collapse
And perhaps most interestingly, will the SNP really take a lot of seats from Labour. The SNP traditionally does not do well in National elections (they polled only 3% more than the Tories at the last election). Depends if the "post-referendum" effect has a lasting effect.