but, who is a typical member of this 13%?

a large percentage of them actually voted neither tory nor labour but ukip in the last election, but historically voted for labour. so, the potentiality of a shy socialist effect is that much more exaggerated. they've been carefully steered from ukip to tory, but are they just telling the pollsters what they want to hear?

but, it follows that the "shy" here may be neither tory nor socialist but ukip itself.

this is all speculation. i'm just pointing out that it appears that enough people are undecided and merely leaning tory to conclude that the decision will be made by people in the ballot box - or by people that decided in the end that they would rather do something else.