As soon as it does, The Conservatives are fucked, because they just don't get that big a share of the vote.
It's therefore in their best interests to show that they can be part of a coalition government. Because electoral reform will lead inexorably to an end to monolithic governments. If they want to be part of the ruling government of the UK in the future then they need to show that they can be good partners and work well with others, or they're essentially consigned to history.
I wonder if that's why they were willing to compromise so far with the Lib Dems.