Thanks very much Gromit...a good stab, which others have failed to do. Just a couple small points.
YouGovs surveys are always about 2000 people, not 1000, and only an average of 14% are "don't knows"
The new element for next May will be the UKIP vote, as they really weren't significant in any way in 2010. All the evidence shows that while they have stolen some votes from Labour, it is the Tories who are likely to lose out the most. That is, of course, if the UKIP vote holds up, and I for one don't think it will. Even UKIP themselves predict that only half of their vote will stay with them in 2015, which seriously discounts the effect that they will have.
The Electoral Calculus website has now updated ::
http://www.electoralcalculus.co.uk/homepage.html
This time last month, it showed Labour with a 40 seat majority next May, so
its appears to be going down. It shows that the probability of Labour having an outright majority has come down from 63% to 57%, but interestingly, it still shows UKIP with zero seats, so much for Farage's earthquake...more local Councillors, and more of its MEPs on the Euro gravy train, but still no representation at Westminster.