I've found where you've posted a link to an article (not the poll itself btw) which discusses a TUC funded poll which asks former former Labour supporters and others who have switched their support and non-voters about the pros and cons of voting Labour (among other things). And the replies are basically text book pros and cons of voting for a center left party. Pros - Labour are on the side of ordinary working people and will improve the NHS, cons - they would spend too much and can't be trusted on the economy, they would make it too easy for people living on benefits.
Non of this suprises me. The author of your article only quotes the cons and concludes from this single result that voters and non voters both question Labour competence in government (ie. Blair and Brown's governments). If you read him carefully he refrains from saying that non-voters have the same overall views as voters. And indeed the statistics hint at rather complex picture where roughly speaking the public are well "to the right" of Labour on welfare and immigration and well "to the left" of Labour in terms of taxing the rich, not bailing out bankers, education, health and investing in the economy. But note - a lot of people were plumbing for "don't know" options.
You should always note that it is especially hard to tell where non-voters stand as there are plenty of people who genuinely don't know and don't care when it comes to politics in general and westminister politics in particular. They aren't likely to be wasting their time with polls like this and are always going to be under represented.
I would just like to add that as somebody who used to be in a left wing party that had locally limited but successfully won over many non-voters, that these are our people, the people that we should be listening to. Sure, they aren't an army of potential Corbyn voters but dismissing them as a particular part of the political spectrum is both ridiculous and patronising. If polls tell us anything it is that people in general do not fit comfortably on a political spectrum. This blather about Corbyn being too left wing is getting tedious. To be sure, not many people share his politics but then not many people share the politics of Tony Blair or any centrist politician you care to name.