Reddit reviews Sex, Lies and the Ballot Box: 50 Things You Need to Know About British Elections

There's an interesting chapter in this book about that survey and public opinion:
http://www.amazon.co.uk/Sex-Lies-Ballot-Box-elections/dp/1849547556/ref=sr_1_1?ie=UTF8&qid=1425402914&sr=8-1&keywords=sex+lies+and+the+ballot+box

They basically talk about studies that show that while people's actual grasp on the numbers is quite poor, there's a direct link between what's delivered and public opinion on that thing - so for example, if education spending increases, the percentage of people thinking more needs to be spent on education decreases. One of the things they mentioned was unemployment - people were way off on what they thought unemployment was, but if you plot the proportion of people who in surveys responded saying that unemployment was the greatest issue over time, it follows very closely the official unemployment figures over the last 40 years.

I put it down to a really poor interpretation of polling data. I've literally just been reading about this "tough on crime" question in this book. It basically says when asked what the biggest problems the country faces as an open question very few people answer crime, but when its suggested it polls very high. What Labour have failed to grasp is that people that have smoked weed or do smoke weed feel very strongly about this to a point that it could very easily sway their vote towards Lib Dems, even if these people are a smaller proportion than those who generally oppose it, not many people would decide their election on crime generally.