There's a big portion of Don't Knows in the polls who appear to be Lab/Lib waverers. They're mostly younger (20-40) women with a past history of voting Labour. At least on the base evidence available, there's not going to be an undecided break for the Conservatives unless something crazy happens - their voters are pretty set.
So, along these lines, YouGov have done some Labour marginals polling.
Now, isn't that interesting.
It looks like Labour could massively outperform their national polling because people like their Labour MPs, even if they don't like Corbyn. And people think a Tory majority is better for the country, but worse for them.