I don't think you could preserve the 20+ leads no matter what.
The Tories did gaffe by their policy releases and getting focus onto them rather than the Government. The debates - well, I saw it pointed out that the last pre-debate poll was very close to the final score in any case.
Maybe getting rid of the expenses thing might help - but at the end of the day, every Opposition falls back from whatever (and whenever) their best position was. There's no iron rule of "swingback" from a given time, but there's always a focussing on the actuality which pulls back a number of defectors from Government support back into line.
So you need something really major right before the election. Like the Prime Minister being found to be a KGB spy, or something ...