As you say, the Tories are quite happy to sit back and let Labour do damage to themselves. I think they'll leave the big attacks on Corbyn until the last two weeks.
This is what they keep saying, but one does wonder how true it is; for a start, almost everything there is about him has already been used by loads of opponents and we have had eighteen months of regular attacks on him by a fairly united political class and media - so there is unlikely to be the huge bombshell out there, and even if there is people are not going to be surprised by it.
This is after all what happened in the US presidential election where the media and the Clinton camp spent months having all manner of legitimate and illegitimate pops at Trump, to the point that [Poor language removed]-grabbing tape was released it had zero effect for them - enough people either didn't believe it (because of the clear agenda the media had in that election) or weren't surprised by it to the extent that they wouldn't vote for him because of it.
If the polls continue to narrow, I'd expect that the attacks will probably take the form of increased internal opposition to him - Blair's increasing visibility lately being perhaps a precursor.